Because a DMG% chip is a share of a total, not a bonus stacked on top. Your damage is base x (1 + turret level bonus + element DMG + chip DMG), and the turret's own upgrade level is by far the biggest term in there: a Lv45 turret already deals about 4.7x its base damage before a single chip.

So adding +50 points to something that big barely moves it. With nothing else in the bucket:

Turret Lv.20 — a +50% chip is really worth about +19%
Turret Lv.30 — about +15%
Turret Lv.40 — about +12%
Turret Lv.45 — about +11%

Element DMG from liveries lands in the same bucket, so the more you already have, the less each new chip is worth. The stronger you get, the weaker every extra DMG% becomes.

Two things escape this entirely: Guardian turret DMG, which is a true multiplier applied at the very end (+225% really is 3.25x), and Rangers, which have no upgrade level in the mix — DMG% chips hit roughly 5x harder on them.

FAQ

Where are the coordinate