Some of it may well be historical - while there are a lot of generic hardware store type things that "do the job" out there now, I'm willing to bet that a few decades ago this wasn't the case.
However, I'd still say that unless you're just doing light hiking / camping where it's never going to be stretched much, it's still worth getting:
- Overall, it's really not that expensive, even for the genuine stuff (especially when you buy in bulk.) OK, cheap knock off cord can be substantially cheaper in some cases, but it's not like proper paracord sets you back anything more than a few pence a foot.
- It's more versatile - you can cut it and use it as one cord or strip the inner cords from it if you need something thinner (or even strip the inner cords and tie them together if you need something thicker.) Can't do that with most other stuff.
- You know what you're getting with it. If you get the same paracord every time you know you're always getting, well, the same ol' paracord. Get cheaper hardware store stuff and chances are it'll change over time - perhaps for the better but perhaps for the worst, and you don't want to find out at the wrong moment in a gale when repairing a guy rope!
- You say the generic stuff is strong enough to be used as guy ropes, but is this in a light scenario with no wind or in a heavy context with lots of gales / rain? It relates to the previous point, but are you sure it'll definitely hold up in those sorts of circumstances? Are you sure? You pretty much can be with paracord.
- It often handles better - I find the outer sheath a lot smoother than a lot of the cheap stuff, and it's easier to coil too.
Sure, you could get cheap stuff for doing odd tasks where it doesn't matter, and save the proper paracord for where it matters, but this is a lot of hassle and IMO the price point doesn't make it worth faffing around trying to save a few pence here and there when you've got a decent solution that can do the job already.