People can be very fickle creatures, and develop angst over the most peculiar of things. We do notice patterns which sometimes prompts us to ask users why they feel they should delete their account. Sometimes, it's just to be disassociated with a certain post - or other things we can do for them without going as far as deleting their account.
When this turns from a pattern of peculiarity into a pattern of abuse, we do have tools at our disposal to slow these people down, and we use them when needed. If you've been question blocked 10 times and had your account removed without showing even a minute amount of improvement - you'll probably find yourself automatically suspended for a while when you attempt to re-join the site.
However, these are still rather rare cases, even at our scale. We do not want to get in the way of a user that wants to remove their account, for whatever reason. We feel that doing so is just icky, if you want to leave and have your information deleted, you should be able to do that with the least amount of hassle as possible. We ask that you modify your profile to verify that you own the account, and we might ask if there's anything they'd like to tell us - but we process the request as quickly as we can.
Once initiated, they have 24 hours to change their mind by clicking a link to stop a timer on their profile, then it's done. I wish other services could be that considerate.
There are cases where we will preserve votes for users that voted many times with impeccable records on the site and just decided that it was time to leave. However, if a user has 1k votes to their credit and has been suspended 6 times for running sock puppets, we're ... probably not going to keep those around.
Note - catching the abusive cases sometimes takes a while, because we do process over 100 deletion requests every day. However - if you're serially sucking, we'll pick up on it rather quickly.