Usually, there is some legacy system behind the scenes that can't be updated because reasons. Not an excuse, but it isn't always that easy.

I wonder if another reason may be the differences between traditional PCs and mobile devices with virtual keyboards. I've seen special characters that are two taps away on my phone that I'd have to look up an Alt code for on my PC.

I think the usual lazy reason is to defend against Bobby Tables[1].

Sites that don't cleanse the user input properly.

[1] https://www.xkcd.com/327/