For me, it's because it's easier to say to the wife:
"I've just ordered a new part for my rifle"
Instead of
"I've just bought a new gun"
She'd go livid if I kept buying guns.
Upgrading the existing gun is good for keeping her sweet.
Plus I'm well happy with my rifle since ditching the standard barrel and hop unit for a Prometheus/Prowin combo, so I don't need a new rifle just yet (saving my pennies for a TM Recoil mind...). Just could do with a better spring in the Specna and it'd be perfect.