Because in many countries, including US, it's illegal to leverage one product to ensure unfair market advantage for your other products, and this is very close to that...

Intel spent money to prevent their competitor from leveraging speedups from their compiler. It would have been less work for them to simply use feature detection on AMD chips, since Intel already has to implement feature detection for their own chips.

It took slightly more effort for them to put in the anti-optimization feature than to do the feature detection correctly.

So the answer to your question is "they don't need to", but your question has nothing to do with the problem.

Because it would benefit the user of the compiler, their customer.