Honestly? Because companies view those kinds of questions as proxies for IQ tests (which are mostly illegal to use as a hiring method). Notice the defense of them largely revolves around "seeing how you solve a problem" and "how you communicate your problem-solving ability". They're seen as measures of your intelligence, which is what they're actually interested in--not your experience or expertise (outside of specific areas).

Those interviews are, of course, something you can easily (with a serious time commitment, but fairly easily nonetheless) prepare for and game, which of course makes them bad proxies for intelligence. And of course, I don't think algorithm questions necessarily measure someone's actual intelligence at all. And of course, it's unclear how highly intelligence is correlated with your ability to succeed as a software engineer.

But that's why. I have little doubt that they would actually try IQ tests if they thought they could get away with it. I'm fairly certain at least one major company tried asking candidates for SAT scores.