(1) You can't assume that there are a lot of equally qualified A players for those companies. In fact, those companies struggle to hire enough qualified engineers.
(2) You're assuming that those companies, if they were presented with multiple qualified people, can only hire one of them. In many cases, they would just hire all of them.
(3) You're assuming that, if a company has to decide between multiple qualified people, it would use personality or looks as a dividing factor. Obviously, the people aren't actually identical, other than looks. The companies could instead decide based on a preference for public universities over equally ranked private ones, or what city the people are from, or whatever. There are many differences between people.
(4) You're assuming that attractiveness would help someone's candidacy for an engineering role, and not hurt it. If you were talking to two people -- one was a very attractive woman and one was a very unattractive woman -- which would you assume is more technical, if you had to bet? You'd probably bet on the less attractive person.