In my case it's cause I wanted something like herdr or cmux i.e. a terminal with first-class agent support, but I didn't like any of the solutions that were already out there, hence I made Muster [0]. I wanted to focus on just the AI-ness of it all, I was previously very happy with Ghostty and had been using it for over a year, and Libghostty allowed me to get a ton of work for free, while maintaining a Ghostty-like feel even tho it's my own project.

[0] https://github.com/amterp/muster