Why is it illegal?

Because it's too much of a bother to deal with when trouble arises relating the shared account.

And it makes it kind of unfair, for example:

If some person gets God Belt who's been playing many years, they won't be unhappy about those other god belts that arnt legit, that are just tons of players sharing a single account, unfair, huh? welp, thats another reason

That doesn't make any sense.. :/

Weird reason, but he means that if multiple people share one account, that
account could have a lot of Qi, thus not being "fair" to those who have built
up their own Qi by themselves.. though you can already buy Qi, so it's not
a solid reason :v

Account sharing produces unnecessary drama (almost without exception), is essentially account trading, undermines most of the assumptions that any fair play (and thus ranking or earnings system) is based on, and is completely unnecessary due to it being free to register accounts. Sharing an account doesn't make it harder for us to enforce the rules on you, but it does make it a lot harder for us to do so fairly without affecting everyone else on your account. However, by making account sharing itself illegal, we dodge the problem of "But my perfectly innocent and probably imaginary brother uses this account too, you can't ban him for the things I did!"

Account sharing complicates things that do not need to be complicated. It is simple, effective, and not an unreasonable expectation on our behalf to make it against the rules. Also it's good practice: never give any of your passwords to any of your accounts or things to anyone you do not want the world to believe is you.