It still feels strange to me, to dance the same routines, against more or less the same couples, in front of the same judges, 3-4 times. Why are there single dances at all? Why are championships and scholarships two different events? Why are there 3-4 levels of bronze? Why are there 6+ age categories for adults?
One of the reasons is that this is Pro/Am. That often means the same pro will dance with multiple students. A number of the students that come with a given pro may be near the same age and level (e.g. commonly bronze and/or silver, and commonly over 35 or 45 or 50 years old, etc). So having all those levels and small age brackets gives the pro the chance to dance a lot of heats with his (or her) various students. And students interested in dancing a lot of heats also have that option, if their pro isn't booked up with other students.
Ditto for scholarships and championships - the pro can dance with two different students, or the same student twice. Age group cutoffs are sometimes different between the scholarships vs. championships, too, so you sometimes see a different mix of couples in one than the other for that reason.
And of course, the more heats the pros dance, the more money they make. And the more heats the students dance, the more money the organizer makes.
Some students like to dance a lot and have the budget to do so.
Some comps (like Ohio Star) have stricter limits on how many heats dancers can do, and have somewhat fewer age groups and sub-levels. This often does result in larger heats for single dances, since it's a big enough comp to attract a large number of pros. (It's my impression that it also results in students fighting over who gets to be the X level, X age group student at that comp, since the pros still often have more than one student in bronze or silver or other levels in the popular age groups - they just can't dance with all of them at once.)
Collegiate comps do the huge heats like you suggest - because they aren't pro/am they can do that. And they also just have ONE age group, so everyone is lumped together, bronze is bronze and that's it for a given set of dances.
(I'm not defending, just explaining.)