I think modern TTRPGS in general steer towards things like temporary summons because of how it lets the players actually use them in combat. Nobody wants to play the necromancer who is suddenly just some guy because there are no corpses available where the battle kicks off.

I have an enormous soft spot for narratively putting in the legwork to assemble your undead hordes, and when I'm the GM, I'm always keen to set up good moments for the necromancer to build an army, but it's so easy for that to set up a situation where a player doesn't get to actually use their features. Making them temporary summons from nowhere in particular is the easiest fix.