Where to fucking begin...
Anyone who says 'last couple of years' has either been living under a rock or is only aware of airsoft trends because forends that extend right to the base of the muzzle device have been a thing in US civilian shooting for many, many years now. As with a ton of things we see now, it was spawned from competitive shooting, because they're the guys pushing the envelope who really, really want to shoot faster and more accurately than the next guy to run a given stage. More accurate shots on target in less time = more money in the bank and trophies etc etc, same as winning in any sport. So that's where it comes from, but the US Army has realised that the concept is the right one, their dedicated marksmanship unit utilise 'long rails' (which are just standard rails if you actually want an effective rifle) and the latest generation AR for various parts of US SF includes the Geissele URG-I upper, which looks like this:
And I promise the guys using those guns are 10-100x better at any shooting discipline than anybody here.
Fact is however you can play airsoft while holding on to your barrel, not like it gets hot, but I've literally never seen anyone do that because we all find some sort of handguard more comfortable. Which style you choose matters naught and the ignorance in this thread is pretty rife, but as I say none of it 'matters' in bbwrz so I'll speak to actual firearms.
Grip and recoil control
A rifle is like piping icing or fighting a fire; you want control then you grip that thing as close to the end as possible, this is utterly basic physics. It is a compromise in comfort and endurance, and the older style of traditional grips generally make more sense the longer range you're shooting, which is a result of the fact that military doctrine used to be all about shooting as far as physically possible. You look at firearms design and usage doctrine from the late 1800s through to today, you see moves from .50+ cal muskets to .40+ cal black powder cartridge guns, then 30 cal bolt actions, then 30 cal (but shorter cases) in semi auto, then longer rifles (M16) in .22 and finally to today where the carbine is king. Shorter and shorter ranges, smaller and smaller cartridges and progressively more of them. Gone are volley sights for 1200m+, en masse firing and single shots, we want large magazines, red dot sights and short barrels to clear buildings and maybe shoot to about 3-400m (in terms of the individual rifle). The next move is polymer/bimetal cases to make ammo even lighter, maybe step up slightly in calibre but that's a long debate. But the point is Close Quarter Battle is the focus and the fastest shooting technique when you're in close is some type of extended grip.
Increasing sight radius for irons is a factor, but irons are basically dead in western military and competition usage. Even in the pistol realm the US Army has adopted the P320 with the option to mount red dot optics and they are waaaaaay easier to shoot than irons on a handgun, it's like turning on easy mode. But for lights and lasers, the closer to your muzzle they are the less splash you might get on to your barrel, which will both potentially highlight your presence to the enemy as well as decreasing effectiveness of that light/beam. Particularly when so much fighting is undertaken at night to make the best of the technology edge our guys have, an effective IR laser is truly paramount. Under night vision even the optics are a secondary let alone irons, aiming is mostly done by laser. Since you're already holding the rifle close to the muzzle for optimal control, the laser now wants to be in front of your hand to prevent splash when shooting from either shoulder and the likes of the old 7" KAC RAS simply do have have the real estate for your hand (from either shoulder), your IR unit, white light and any pressure switches; not to mention perhaps bi-pods and foregrips. If you're seeing people using long rails but only attaching stuff/gripping right back near the receiver, all that means is they're not utilising the rail properly... or it's bbs so it doesn't matter. The people doing real shit however, do make very extensive use of all that rail space.
Personally I shoot airsoft guns the exact same as I shoot real ones, I also set things up in ways that are very much well proven by better shooters and also ergonomic and make sense to my mind. Seeing as inner barrel length make so little difference in our low power smooth bores I much prefer shorter barreled guns which translates to only having perhaps a 9-10" handguard. That gives an extra bit of space to both grip the gun from either shoulder and mount both a light and pressure pad along the 12 o/clock rail. There's also the capacity to add a PEQ/DBAL/MAWL in to the mix down the line for any event when I'm using my own NV. Extension of the arm compared to 'traditional' technique and equipment is not changed significantly since the rail is only a couple of inches longer. I'm also pretty OCD about checking the weights of any forend (+mounting hardware) I buy and I have a couple in magnesium/aluminium alloy which extend to the end of a 14.5" barrel yet weight far less than a 7" KAC RAS.
Costa has been irrelevant in tacticool culture for easily 3-5+ years now and he turned some silly shit up to 11 that created some un-evidenced bias about certain ideas amongst closed minded people. There is also massive part of this discussion entirely missing which is the difference between free-floated barrels/handguards in the real world and ones that do not freefloat, not to mention the huge differences in quad rails and KeyMod/M-LOK but the latter should be entirely self explanatory if you've ever spent any time looking at pictures of guns; real or fake.
As I've said in a dozen threads here already, if you're genuinely getting a red face and a throbbing forehead vein over "why do people do XYZ???" you need to just stop and realise we're all playing a stupid kids game and literally none of it matters. Nothing anyone else does necessitates copying, we can all use an inner barrel as a BB blow pipe or a replica Proton pack with an HPA system and lead weights inside if we like.