As technology facilitates pushing this practice further up in the hiring pipeline, even to the point of initial contact, this presents a grim prospect for anyone who is already facing career headwinds due to ageism, let alone any of the other -isms. Ageism is a threat to to everyone regardless of gender, race, national origin etc. The makeup industry is going to be thrilled.

If there's a vast under-employed pool of people of the "wrong" age, race, sex, etc. because "bad" companies refuse to employ them, why don't their "good" competitors seize the opportunity and hire all that untapped talent being left on the table?

Many people, even those making hiring decisions, aren't in the entreprenuerial min/max hustle culture. Those employers will consider factors other than skills and the lowest agreeable wage. People tend to like other people similar to themselves so that leaves various undesirables unemployed longer.

The set of potential employers is sometimes limited, especially in the before times when people would only look for work for up to about an hour commute from where they live. Also filter employers down to an industry/skillset. Then filter by employers with openings that coincide when an applicant is looking for a job. Consider two dental technicians in a rural area that start looking for a job on the same day. If one of the three dentists in the region has a preference for some protected class, then the "wrong" applicant may be job hunting for a while longer.

There are also some jobs with tangible incentives for discrimination. Any role that benefits from sex appeal can favor young, attractive females. Jobs that are physically demanding can favor young and fit men.

Most markets that are important to you do not adhere to these constraints.