That's because you choose to. You could easily move to mbin, for example. This post made me realize I had been holding back on it without a good reason.
Not listening to to community. We are the content creators on reddit. Reddit should have done as we asked. They threw us under the bus.
Lemmy devs dont listen to their community. Instance admins point out serious legal issues regarding moderation, and they say they don't fix those bugs because user privacy doesn't matter.
Unfortunately there was some miscommunication in this issue and we failed to get to the root cause. In fact the Lemmy backend has an option to delete all content when an account is deleted. This used to be the default behaviour but was changed in 0.19 so you need to set a parameter delete_content. We failed to add a checkbox for this parameter to lemmy-ui.
However the checkbox is added now in #2385 and will be included in the next Lemmy release. Other frontends and clients may also need to adjust the delete_account api call.