Why are summoners so neglected?

[Shougun]

I mean, if we want to use WoW as the example, pet diversity was a complete waste of resources early in the game.

Yes and no, depends on the patch and the content. There were niche summons that were useful throughout for very specific things, so during generic content usually a very specific right choice and a lot of wrong - but could vary depending on strategy and situation.

At its current iteration its a zoolock and so you get to use a lot of pets at a lot of times lol, so at this point there are a lot of right choices and its more like picking the right order.

If they added more egi types then Summoner could greatly benefit from cooldown summons which would increase their usage, but even not you could still build a few more niche summons that would perform better during their niche - although I imagine most people would ignore it and use the generic "usually" good choice, and the raid group would enforce you to use the right choice. WAAAYYY nerfing Fenrir and turning it into a temporary cooldown summon (from FFXI) could be functional random example, doesn't even have to be a buff - summon diablos for damage temporarily.

So I would argue it was not a waste if you wanted to focus on /many/ pets as Warlock in WoW has some of the top sustained single target DPS (in fights that allow them to whip their pets out, lower movement fights for example).