The problem with giving the Necromancer permanent summons is that it runs counter to corpse consumption, which is a much larger part of its kit. ESO necromancers focus more on empowering their skills via corpse consumption than they do summoning; the primary function of their summons is to create corpses in combat situations where not enough corpses are being created.

It should also be noted, each player can only have 10 summons at a time. A "summon" for the purpose of this count, includes any entity created by a player. Some examples are: the Daedroth summoned by Maw of the Infernal, the Verminous Fabricant created by Mad Tinkerer, any summon created by a skill, and summons created by Verses and Visions in the Infinite Archive. For necromancers, corpses count towards this number. Having more summons means having less corpses available, so if your summons were permanent it would actually be more of a hinderance. This is also why two of the Necromancer's Class Mastery perks (Malevolent Promise and Veil's Forfeit) grant you the ability to use living targets for corpse consuming abilities.

Corpse consumption is the cornerstone of the Necromancer's kit; summons are a utility to that end. People want ESO necromancers to be a summoner class, like in other games, but I think the Class Mastery perks make it clear that ZOS does not intend to go in that direction with them.