I think very few people proactively decide to become partisan. Most liberal arts academics just want to work in their special field. Sometimes the things they study get politicized, but that’s mostly the doing of talking heads.
If somebody wanted to become a partisan hack, there are easier ways than getting tenure, right?
It's not so much that people want to become partisan, but rather the culture of the discipline.
The culture within the hard sciences is to challenge existing theories and narratives.
The culture within the social sciences is almost the polar opposite. It still superficially presents as a science, but in practice is a purity spiral with an orthodoxy of established conclusions which cannot be challenged without severe career consequences.
The hard sciences had their Galileo affair 400 years ago, the soft sciences are in the midst of theirs right now.
If somebody wanted to become a partisan hack, there are easier ways than getting tenure, right?