Apart from being slow, having discoverability issues, not being able combine filters and actions so that you frequently need to fall back to shell scripts for basic functionality, it being a complete PITA to compare things between accounts / regions, advanced functionality requiring you to directly edit JSON files, things randomly failing and the error message being carefully hidden away, the poor audit trail functionality to see who-changed-what, and the fact that putting anything complex together means spinning so many plates that Terraform'ing all your infrastructure looks like the easy way; I'll have you know there's nothing wrong with the AWS Console UI.

Probably, it's also why Google has 100 different chat clients.