Because it’s yet another example of Google’s near-monopoly over the Web’s architecture. It’s not healthy for good web development. It’s like the 90s and Microsoft all over again.
I mean, fuck, we’re already getting websites that’ve been “optimized” for Chromium-based browsers—in other words, semi-broken for non-Chromium browsers.
I said a near-monopoly. Also, even if it’s foss, by creating the format, they established the baseline parameters of that format.
That gives them a significant degree of control.
Edit: I also hate it because so many of the programs I use don’t support it, so I constantly have to copy > paste into image editor program > Save as PNG.
Though admittedly this is mostly an adoption thing. Still, it’s a major problem.