pip is a perfectly usable package manager and is included in most python distributions now. Is it perfect? No, but it is good enough for every team I have been on.
Putting aside the speed uv has a bunch of features that usually require 2-4 separate tools. These tools are very popular but not very well liked. The fact these tools are so popular proves that pip is not sufficient for many use cases. Other languages have a single tool (e.g. cargo) that are very well liked.