I know its a little late to answer but in case anyone in the future sees this and has the same doubt as you.

It just takes a little while for them to get Stronger, after you get the Lich it makes it much easier as a Necromancer, i'm now lvl 36 and my minions are just steamrolling entire maps alone, 200k HP bosses? done in less than 30 seconds by the minions.

Its kinda obvious but focus your entire loot on Minion Damage/HP, nearly all my set has it and i have Minion Enrage/HP passives at max lvl for my character level, the stronger minions as far as i can tell never die fighting, they just run outta time, you can throw your lich in the middle of 5 bosses and he will walk away like nothing is happening, same for the Fury and the Dragon, i saw my Dragon hitting for 20k Fire Damage in a single hit, i don't know what attack he used but its rare to happen but it does a lot of damage.

I just rolled a necro as well and feel the same way. Thank you for the reply @NouH !!