You definitely could (and, indeed, I can imagine beneficial use cases for a "renice" feature in a browser like Chrome with its Task Manager).

But there's less of a significant need for it. Main CPU performance is too poor on hashing datasets to much benefit from salami-slicing user CPU at scale. That trick is much likelier to get you a solved block in the blockchain if you can tap user GPUs on a couple million users.

Yes, hence why extensions like NoScript and uMatrix exist(ed).