Part of me wonders whether this isn't exclusive to dating, but a lot of fields and things come the internet, globalisation and social media.

Perhaps that's why getting a job seems to be much harder now for a decent percentage of people too, since now the competition is worldwide and mostly sky high. Employers can keep looking for the 'perfect' employee (and skip over people they would have hired with fewer options) and employees can keep looking for the 'perfect' company or organisation (and skip ones which may have been 'good enough' before).

The market's been widened significantly, so those who aren't at the top of their field are against much tougher competition.

Yes, although there is no “monogamous” assumption with employment, so just as you would switch to a different vendor offering the same product for cheaper, so would employers. It doesn’t seem employers have been hurt much at all by the new dynamic, especially as more and more work becomes automated or broken into simpler routine tasks.