I wasn't there when that decision was made, so it's hard to say, but there's good reasons we tend to re-implement stuff found in existing open source software: Designing and building our own lets us be faster for our specific use case and gives us the control to fix bugs that are performance related or otherwise. It also means we hold a full understanding of the stack in-house allowing us to fix a lot of things. The most painful bugs for us to fix usually come from things we don't have direct control over, like OpenGL drivers, system libraries, etc.

As a hypothetical example if we used GTK instead and it's rendering implementation was designed to be CPU based - say it's based on direct pixel manipulation by passing around a texture - if we wanted to be performant at high resolutions we'd have to refactor/rewrite large parts of GTK to use GPU rendering instead. That would mean either working with the maintainers to get something they'd be happy merging, then doing the work for all of gtk, or forking gtk ripping out the parts we don't use and just refactoring those. The latter would be significantly less work and the likely choice, so we'd likely end up in the same place as before.