The reason why you cant have more summons is because of a single design philosophy: "everything must be about muh balance"
Its a dumb and stupid idea that should have only ever applied to MMOs, but sadly some dummies decided that it has to be the be all end all of all videogame design.
Now you know what to blame.
Even on mmos, Age of Conan allows you to have up to 8~11 minions as a necromancer. 11 by taking some disadvantages and your mininons get the "minion" template which makes then weaker than the regular version. And note that AoC is not a old school pre WoW mmo, where fun and immersion was above balance. Thin on ultima online times.
