Although, I think the statistics would be very interesting to read over.
Because most good ideas which people are committed to aren't "get accepted into YC". Why shouldn't someone say "I'm not likely to get accepted into YC, so I'll spend my time working on my good idea instead of filling out the YC application form"?
they don't have a standard spots available number.
Correlation does not imply causation. Repeat with me: correlation does not imply causation
Correlation does not imply causation -- but correlation can still be useful for making predictions or decisions. The fact that the word "V!@gra" appears in an email doesn't cause the email to be spam, but it does provide a strong indication that it's probably not worth reading said email.
If (taking a wildly hypothetical example) it turned out that 50% of YC applicants were over age 30 but 0% of the accepted YC applicants were over age 30, it would a strong indication to potential applicants over age 30 that they needn't bother applying -- and this is true whether the YC management has a deliberate bias against over-30s or not.