Some accounts are deleting their posts after a few downvotes. It's devastating on communities like c/asklemmy.
Lemmy doesn't track an account's karma like reddit. So, all downvotes will be isolated to your post or comment and won't affect your account—unless you wrote something truly horrible.
Remember lemmy is a community effort. Deleting a post also removes all comments on it. So you are not only robbing the effort others put in, you are actively removing knowledge from the fediverse. Others won't be able to find it through search and lemmy will seem lonelier than it already is.
This really needs to change. OP shouldn't have control over the conversations everyone's having, even if they can remove their portions of it. Even reddit didn't give OP that power, even if it was removed from the sub page.
FWIW some instances/front ends preserve the comments section, although it ends up being a bit jank. For example, if you load this through lemmy.zip, it shows as an error because OP deleted it. But if you go through old.lemmy.zip you can still see (most of) the comments, although not what the original post was.