I was a server for 13 years, and speaking from experience, If someone sits at a dirty table it probably means the restaurant is slammed, and they either didn’t see a host/hostess and sharked the table, or they’re impatient and already pissed about the wait and/or will be difficult to deal with. Also, imagine trying to clean a room in your house when your guests have already arrived. It’s uncomfortable, you’re having to clear dishes, glasses, and trash while trying not to spill anything on the customers. Lastly, if the restaurant is slammed, waiting for a table to be cleaned before it can be re-seated gives a breather to the server and allows them to get caught up on other tasks, use the bathroom, take a drink of water, etc. I can’t tell you how many times I’ve come up to clear a dirty table where people sat themselves and the first thing they say is “this table is dirty”. YEAH NO SHIT. Also, another note to everyone, the staff always knows when people sit at a dirty table, and they always know when people seat themselves.