An answer to this question is in the
Guide to RYL.
A couple of things - some websites do allow account deletion, some don't. One should always assume that things they post on the web will be stored somewhere, and nothing is truly deleted.
A couple of reasons as to why mods won't delete accounts - firstly, people in a particularly vulnerable state may wish to delete their account because they want to close off from the world/from their friends, which puts the moderators in a difficult position when trying to act in the best interest of members.
Also, it creates a lot of hassle if people choose to delete their account and then want it reinstated/come to expect the moderators to do that.
As others have mentioned, there are a number of other options if you do want to switch off from RYL.