Because the fantasy was being someone the readers could identify with until the time came to save the day.
In-universe, of course, there was no reason. In the very earliest issues, Billy used to turn into Cap, put on a tux, and take Beautia Sivana out for dinner and a show. He LIKED being an adult.
That was when Billy and Cap were almost written as separate people. Once they decided he was still Billy "on the inside", beautiful women made him uncomfortable to the point of stammering.
Kid Marvelman stayed "super" for years and became sick and twisted by a superiority complex.
Erik Larsen dealt with the problem by requiring Mighty Man's "host" to eat or die of starvation. >>101377775
Yeah, Billy still went to school -- somehow sandwiching in his job as a full-time radio reporter. During the "two separate individuals" stage, he whispered The Word during an exam and a ghostly specter of Cap appeared to feed him the answers. So even Billy wasn't 100% pure.