It seems that the combination of little "stacking" avoidance ("stacking" avoidance and resolution is weak, aka, processed pretty late and not aggressively, leading to things like large numbers or things being able to be in firing range, much more than their total physical space would normally allow them to) combined with a relative lack of hard hitting AOE stuff means that there is little to discourage blobbing, either with mixing ship types or blobbing a single ship type. There rarely a need to be considered about positioning and "splitting" your fleet up at least some.
I noticed, taking SC2 as an example, almost all of the "blob punishing" AOE stuff is an active (manually triggered, typically with cooldowns and/or an energy pool for them) ability. If it was just a normal weapon with no downsides, it would almost certainly be OP if it did enough damage to each unit to matter, or UP if it did not. (Deployed siege tanks don't count. Yes, it is a normal weapon, but only when they give up their mobility)
AI War has had a long running "tradition" of not having active abilities for normal units. Only big, important stuff (right now, only the champions of the 4th expansion) have any hope of getting active, triggered abilities. TBH, I would like that to stay (IMO, nothing less than a spirecraft level unit should ever have an active ability, and even that is pushing it. Golems I could see), as I think this is one of the things that has helped keep micro a lesser importance in this game.
Could this be solved by adding more AOE threats for the AI (or buffing up the ones it already has)? I don't know.
Could the "stacking avoidance/resolution" be more aggressive without killing the performance of the game? Again, I don't know.
Could making the AI be much smarter about its fleet tactics (kiting, splitting forces, moving things away from their strong counters, etc), showing off how effective fleet tactics can be, may start pressuring humans to do the same to be able to keep up? Once again, I don't know (not to mention this last one would be a nightmare to code).
I just wanted to point out some observations.