Probably because discord will send a text to the number to verify it. Spoofing a number is easy as long as you don't care about receiving replies at that number.

You can, just register prepaid phone line with top-up cards purchased in cash from a store. If you use a Google Voice or other VOIP service, this shows up as VOIP and they won't let you use it. Annoying but it's the cheapest/easiest workaround I know of.