When programming on the GBA there are tradeoffs to everything, and if you are going to want hundreds of ROMS with hundreds of characters (as opposed to the 50 something you have now, up from 8:3 of older eras) you brush up against read speed limits, RAM limits, CPU limits and more all in your kernel that most want to spend as little time as possible looking at.
The ? in one stuff has long been a problem though. Remember having to redo things a lot on the various lists I used to maintain.
On not being released as singles then yeah there were a few that got bundle or nothing type setups. I will say though that there are a few patches out there both to remove the health and safety screens so often seen on them (yeah they appeared in ROMs on the later GBA) and also at least make it so a give game autobooted (might not be a smaller ROM but you did not have to have a flash cart and then another selection). Making autoboot patches is also not the hardest thing in the world most of the time, but enough that you are starting to get your hands dirty in assembly.