Because it's good PR, customer service etc and you never know, they may not find anywhere suitable in the end and come back to you because you answered them quickly and politely and that put you in a good light amongst the rest ... etc etc...but I don´t see why I should waste my time answering when I know that my property is totally unsuitable for their needs.
I don't even bother paying attention to calendars when I make an enquiry because I can't trust the veracity of them.