The hardest part of summoners in Titan Quest is the normal difficulty. Once you get past that into the next difficulty, the summons receive huge HP scaling bonuses so they can actually live for a while.
Sounds like a stupid difficulty design to me when the higher difficulty actually is easier to play.
Summons are the only case in the game that applies to. It has been a big topic ever since TQ released. The problem lies in the fact that the difference in HP scaling compared from Normal level 1 to whatever level monsters start Epic at (40 or so) is a massive jump (several times the survivability). The summon scaling is always performed at the start of the difficulty and it is not nearly as massive when going from Epic to Legendary. Due to that, the difficulty progression for summon builds is 2-1-3 or 3-1-2 (Normal-Epic-Legendary, 3 being the hardest). Act 5 is notorious for absolutely deleting all your summons constantly on normal. The person who started talking about it even mention the Charon boss fight, that fills a small area with multiple extremely damaging AoE attacks that delete even characters standing in them for a couple seconds, not to mention unscaled summons. NOTE
: I am talking about TQ Gold. No idea if they changed anything about this in the Anniversary Edition but from the sound of it, doesn't appear so.
Because you cannot (and couldn't, even it would be possible) choose a higher difficulty level just to start a game with? It would be borderline impossible to progress at Legendary with a LV1 character.
Of course it's up to you to define what difficulty is for you, but it doesn't sound like it like that definition would perform well...
Difficulty levels - several options where a higher one is more difficult than the previous. Ignoring the edge case described above, it fits the definition just fine. When it unlocks is not really relevant.
But let's just agree to disagree.