Because they are universally incapable of coming anywhere close to the full power of git.

I can't tell you how many times I've had GUI-only people ask me to unfuck their repo (fortunately not at my current job, because everyone uses the CLI and actually knows what they're doing). It's an impedance to actually learning the tool.

Ultimately any GUI is a poor, leaky abstraction over git that restricts many of the things you can do for little actual benefit.