I feel like Larian has this really, really weird phobia of summoning / pet effects. Summons and animal companions are generally a fairly weak option in 5e to begin with, yet they've completely gutted them by limiting you to only one at once - something that completely guts the Ranger and Druid in particular, whose classes are very much designed around being able to summon multiple things at once. Concentration already exists to limit powerful spells, after all.
It's comparable to someone saying "oh, the fighter getting multiple attacks a round feels unbalanced at a gut level, better limit him to one."
