I'm guessing guided trips start super early for a couple reasons. First, things can get more dangerous from sun exposure, such as snow bridges softening up, wet avalanches, cornice and serac collapse, etc. Its also easier to move when the snow is firmed up as opposed to slushy and deep.
Second, big guided groups include people of different abilities and some people may be really fucking slow.
Finally, if anything did happen and someone got injured its better if its noon instead of 6:00 p.m.
So a competent, fit person probably wouldn't have to leave Muir at 2 a.m. to tag the summit unless they just wanted to get done early.