Why don't sitters respond?

If I request info from a sitter that lists the time period I'm looking for as available, why do so many not respond? I have sent out approximately 12 requests over the past three days with detailed information - I got ONE response that told me she was unavailable and two others with request for M&G,

That seems a low percentage. I don't understand why a sitter would not respond to just say "I'm not available." Or change the availability on their calendar. This seems a lot of extra work on my part, when I can just take them to a boarder.