Why aren’t these things considered Trilobites exactly? They look like Trilobites and I’m pretty sure they fill some of the same general niches? Is it just a taxonomical thing? Are they just not in the right clade?

Two things: first, horseshoe crabs almost certainly have changed/evolved considerably over four hundred million years, just in aspects of their physiology that aren’t fossilized. Second, horseshoe crabs have like nine different types of eyes; even that tail is essentially one big eye, covered in photoreceptive cells. We humans consider ourselves the “dominant” species, but I don’t think we could handle crawling around in slime and mud for four hundred million years quite as well as they have.