It’s because Chrome is so ubiquitous. You go to any Google site, particularly search, in a browser other than Chrome and you’re presented with notifications to install Chrome. Plus, its integration with Google accounts presents a great value proposition for many users.
Personally, I was on Firefox for years until I got a MacBook Pro in 2014. For whatever reason, Firefox would constantly crash, so I switched to Chrome. I only went back to Firefox in about… 2019(?) when they released Quantum and I’ve been on it since. It’s really the perfect browser, particularly with extension support. I also like how on Android you can install uBlock Origin.
With Google’s impending Manifest V3 looming on the horizon for all Chromium-based browsers, it just further cements my decision to remain on Firefox. I do keep Brave around as backup for the extremely rare situations where something for whatever reason doesn’t work in Firefox, but that’s becoming exceedingly unnecessary.
For me, it’s not image viewers, but websites that take photo uploads. None of them that I’ve ever used have supported webp, so I always have to convert to png or jpg.