Why don't employers just administer IQ tests directly instead of using proxies like educational attainment?

It's obvious that most of what you learn at university in say an electrical engineering or computer science degree is entirely useless. Instead, most of your skills are learned on the job, so the real bottleneck is learning speed and critical thinking skills (facets of IQ). Seriously, a 145 IQ engineer with no degree trained on the job is going to be a far superior engineer or software developer than a 120 IQ bachelor's degree