You don't need to upgrade a marker out of the box
You can upgrade to improve something, or for preference
A barrelis an easy option. You could just have a basic barrel and switch to a 'better' barrel.
The basic barrel is there to suit a multitude of players across the world and should work in the heat or cold. A player can then optimize for California, Canada or southampton. They also all have different priorities - efficiency, noise, precise match to paint, over boring, under boring.
You could spend £400 on a gun and would expect something better then the most basic barrel, but to expect the manufacturers top of the range barrel would be pushing the price up, and a wast of money if you already have your full freak, hammerhead, etc barrel set. Let alone if you just disagree with the manufacturer on the optimum barrel
With any other part it's the same - tweaking or tuning, lighter efficient bolts, soft nosed bolts, re-programmable boards, or just needing to have something different from the next man
People also have new money burning holes in their pockets every month and want shiney new things
Some money gets frittered away on mortgages, rent, food, petrol, other money gets to be spent on going out, buying shiney stuff, upgrading and even sometimes playing paintball