If she were half asleep when she says this, I'd say it was nightmares/sleep terrors.
They are thinking more and their imaginations are much more active at 2 1/2 than ever before - which is why nightmares can really kick in at this age.
But if she's seeing them everywhere all day long and when she's wide awake - that kind of throws the whole 'nightmare' thing out the window.
If she's pretending, you'd think she'd have better control of it, and then there's the possibility that maybe she's hallucinating - which you should probably have a doctor check her out for - although I'd say she's probably too young for that.
I'm not sure playing along with it helps any.
Stomping on things you can't see makes it seem like they are really there, but you don't want to ignore her fears either.
Maybe you can tell her you're sorry she's afraid but you don't see any spiders and she is perfectly safe and needs to calm down?
Would be nice if that worked but somehow I think it probably won't.
You should talk about this with her pediatrician and see what they recommend.