A lot of times, there's much more going on in interviews than assessing technical competence. Some of the things interviewers might be looking for include the ability to communicate, ability to translate technical information into something other people can understand, how the candidate reacts under pressure, their thinking process, how they go about solving problems, their composure during the pay/benefit negotiating process, and more. It's incredibly disruptive and expensive to hire a bad candidate and increasingly common to encounter companies with a "no jerks" policy, regardless of how smart someone is.