I really dont understand why its not possible to create summoning builds.
The main issue, I think, is balance
The problem with summon builds is most builds progress fairly linearly, while summoner builds by nature progress geometrically, because you get bonuses to your summon stats, which get multiplied by your number of summons. A melee character, e.g., can only have 1 or 2 weapons that get your per-hit bonuses, but a summoner could have 6 or 7 summons that all benefit from all of your bonuses.
That makes it really tough to implement it in a way that keeps it balanced at all points. If its balanced at min-max endgame, then its going to suuuuuck to level it up. If its balanced while leveling, then its going to be crazy OP when min-maxed at endgame.
Then, separate issues, pathing calculations and AI stupidity
Thats why I think a turret summoner is a better approach. You can design it from the ground up to work with summons bonuses and a lower cap on max simultaneous summons, and it makes it more about how you use them and not just what you have. They're more like static aoe spells with a different mechanic, then, and not like NPCs that play the game for you.