Let's say you're an experienced developer. You have an impressive work record, open source contributions, and ace the technical interviews.
Then your potential employer asks you to take an IQ test.
Legality aside (and it may be framed as a "general aptitude test" or somesuch), this may be very insulting to some people, who feel their contributions and achievements should stand on their own without bullshit tests. It signals to them "we don't care what you've done, and you can prove you can do, or your years of experience or education, you getting the job depends on an arbitrary test we're going to throw at you". You're not going to be treated as an individual, but rather a set of points on a score sheet.
Now you may be right or wrong as to whether these tests are indicative of ability, but for a lot of applicants they would be a red flag.