Most male readers are, for a lack of a better term, losers that rely on escapist fantasies to get by everyday. And while they do want to self-insert as a cool male mc, a male character that looks and acts too cool can take them out of their immersion because they feel like they can't possibly compare. Therefore, the self-insert's ideal male mc has to be a plain jane in everything, so the reader can self-insert better without feeling self-conscious about it. This is why a lot of isekai mcs have typical dull designs and personalities.