There's a character restriction, 4000 characters. But I don't agree that there should be a stricter limit. Sometimes (rarely) it's genuinely necessary to have a long answer, for an issue that is complex.
Obviously, I have some level of vested interest in keeping things as they are (or even making the character limit larger!), but most people don't notice the limit, so I'd say that on balance it's already set at about the right place.