Think about it that way: all the information that sits in your account is still "yours".
Banning you means that you can't do anything that has effects on other users.
But you can come in and check on your content. Whereas deleting your account deprives you from doing so.
In other words: a ban is visible to others, and it prevents you from any kind of interaction with other users. That prevents the banned user from causing further harm.
But as said: their "content" and the information around that, is still "theirs". There is no point in taking that away from them.
And from a philosophical point of view: I think there are examples of users that received serious bans, but who learned their lesson and asked for the ban being lifted. To then become very helpful members of the community. With "capital punishment" (account deletion), that would not have been possible. But this is kinda hearsay from my memory.