Why is it hard to find people with stable mental health on online dating?

Me (33M) have been on online dating for the last ten years. I have dated a few women and been in maybe 3 relationships odd OLD in that time. My relationships or dating from OLD usually end due to the person I’m dating having some sort of mental health issue that I didn’t know.

For example, I was dating someone for a month between November and December, she asked to be bf/gf after only three dates. Then she had fairly minor issues like not getting the result she wanted in her degree. Didn’t talk to me for two days, called me and said she isn’t in a space to date. Prior to that she wanted to book holidays together, be bf/gf etc, sex constantly.

Since then I was talking to another woman for weeks whilst she was on holiday, few days before we were due to meet she said her life is upside down and she can’t date etc.

I went on a date in January, it was good, we arranged a second date. Next day I texted her, how are you? ‘Horrible, everything is horrible etc’. Clearly didn’t want to talk to me and I never tried contact again.

How do you screen for this? Happened too often.