We use 'those' as an anticedent of 'who' instead of 'them', why?

A little vague, but:

"Those" and "who" are both subjects. "Those" is often shorthand for "those people."

Those (people) who run with dogs will get bitten.

Those running with dogs will get bitten.

"Them" is the object. Jack hit the ball to them.

You can't say "them people" as a subject. Them people hit the ball. Emotion: no.

I hope this helps.

anonymousWe use 'those' as an antecedent of 'who' instead of 'them'. Why?

Emotion: tongue tied

Who told you that? It sounds wrong. About what sentence is this claim made?

There are plenty of them who are taking care of business with very small money pots.
A lot of them who ran away ran because they were suffering even more than others.
I was one of them who felt powerless until I had had enough about 9 months ago.
It is them who will pay the ultimate price for these malicious operations.
However, let them who are pure in heart and mind throw the first stone.