Main pg_vectorsearch contributor here. There are multiple reasons why simply “sending a PR” is more difficult than it sounds.

As a company, we want to move at a different pace and try things that a project like pgvector wouldn’t necessarily allow (and rightfully so). pg_vectorsearch is alpha-level software, while pgvector is stable. Releasing early is a way to make it available to the community sooner and getting feedback.

Then there are all kinds of technical reasons: different architectures, different implementation choices, etc. There's also the freedom to try new things and going in completely different directions.