The issue is that the Thirteen were developed in a slapdash manner. A lot of the roster was plucked from previous fiction (oh Prima is the first Matrix bearer in Marvel? Let's use him. Oh Alpha Trion was really old and wise in Sunbow G1? Let's use him).
When they canonized the "definitive" roster they did that and came up short of thirteen names. Solus, Micronus, Onyx, Amalgamous, and Quintas were just created out of nothing to fill out the roster. And Hasbro wasn't going to put off the launch of the Thirteen to seed five new characters in new shows or comics or whatever to slow burn the concept.
tbf they did try. Micronus got some shine in IDW1 and RiD '15. Onyx seemed to get something significant in IDW1 until it was revealed he was Shockwave the whole time (one of a few things that turned IDW1 from something I was a fan of into something I was eyerolling at). Quintas pops up in EarthSpark. Solus herself doesn't get much but I'd argue Windblade's arc in IDW1 does a lot to flesh out why she's important even if we don't get much info on her personally.
Again, the IRL inspirations for the Thirteen are themselves broad stroke archetypes so... like... I don't think making each one a well rounded character was necessarily the goal? They're archetypical legendary founding figures. Background lore.