Eli5 Why is account sharing an issue?

Matchmaking integrity mostly.

Because login credentials are used to identify one unique person. When sharing is allowed, it no longer becomes possible to know who authorized things such as purchases, trades, who played games or got a punishment, etc.

Plus, it's the same virtually everywhere online. One account is only to be used by one person.

Boosting. Do you really want a Silver in your Diamond game?