Contract negotiations fall through all the time, and the realtor sure as hell doesn't want to delete the whole listing only to type the whole thing up again a month later. You could argue that there should be a toggle that just makes it invisible in the MLS listings when it's contract-pending, then brings it back if needed, but 1.) lots of people will still put in an offer even knowing a house is under contract, because they, too, know how often contracts fail, 2.) it doesn't bother the realtor to waste your time looking at a house you can't have, and 3.) if you do notice that all the houses you want are off the market, you start to worry that houses in this area are going fast, you won't get the one you want, you'd better jump quick on the first one you see that is still available, all of which saves the realtors time and increases their commissions because you'll be too afraid to lowball during your own negotiating process.