Honestly the list of ones that DONT use Discord would be shorter. It's extremely unfortunate. I championed Discord for many years as well.

That could potentially open them up to legal problems. Whether it's technically legal or not, nobody wants the possibility of their livelihood being taken away by court costs just because some idiot who is wrong wants to fight them and lose anyway, because they can afford it and you can't (and often times they know it).

I once paid for access to a stock options trading group, but they only used discord. Their website had no other contact info at all. My discord account got randomly banned (it happened right after I joined an innocent server, but maybe because a bunch of people were joining at once, that triggered it? idk), so I could no longer use the service I was paying for. The service auto-renewed on my credit card and I had no way to contact the people to cancel my account (couldn't even make a new discord account). I had to dispute the charge with my CC company and it took months of back and forth with them because they simply could not understand that I could no longer access the only method of support that they offered.