The problem with browsers is that they rely on other standards. If a browser needs to maintain backward compatibility and requires an evolving standard that ALSO requires backward compatibility, this acts like a multiplier on implementation complexity. This also means it becomes increasingly difficult to spin up a new browser engine, and the fewer of those there are, the easier it is for the big ones to just add what they want and have it become a standard, reinforcing the issue.
XSLT adds ~100k lines of specialized code to browser engines with near-zero telemetry usage (<0.01% of page loads), making it a prime example of the maintenance burden vs. utility problem you're describing.
