Blue
Ravenous Bugblatter Beast of Traal
If that were actually true, then why are those people not playing 3.x or Pathfinder? Those games have a much deeper and richer character optimization minigame than 5e could ever hope to provide.
I'll take a shot at this, just for me.
First, someone can like the 5e rules and how they have evolved, improved, and streamlined since 3.x. Or maybe you don't like magic items being a defacto part of character advancement that's baked into the math.
Second, because you want more complexity then ruleset A doesn't mean you want ALL THE COMPLEXITY. It does not need to be a linear - more complexity = more enjoyment, it is more likely "I like a complexity of around N out of 10".
Third, the character optimization in 3.5 got so deep that it's inherently broken because of interactions when you take things from lots of different (official) sources. The ur-example is Pun Pun, but I've played characters that by build could have laid flaming waste to any imagination of balance between characters - except that I didn't. I de-tuned characters and played them to match the power level of the rest of the party. Or if I was above it, it was because I played "#1 support character" - the 4e Defender that allowed all the glass cannons to shine, the 3.5ed party buffer from hell, etc.
Fourth, 3.5 complexity already required lots of pre-planning not to fall behind. So you had to build your character out ahead of time because of all the requisites for feats/feat chains, PrCs, etc. Missing a BAB requirement by a level means delaying a feat for three levels that means delaying a PrC which leans yu don't have the right skill cap to enter another PrC ...
Fifth, and related to #4, finding out that 90% of the builds were seriously sub-optimal compared to the state-of-the-curve of what builds could do, really limited your options.
Oh, and my personal - after running 12 years of 3.x building monsters and NPCs using the same building blocks (and that complexity) as PCs is a crock. The amount of time it eats up is ridiculous. Much rather a "here's around what makes sense for this level, let me tweak to customize".