Why do some people use "that" instead of "who" for a person?

That bothers me too. I was upset to see a movie titled The Killer That Stalked New York (1950), but it's right, "that" not "who".

Or 'there's' instead of 'there are' when referring to a singular. 'There are a lot of countries', not 'there's a lot of countries.'

Or "less" instead of 'fewer." Less is a measurement of volume. Fewer refers to individual items. 'There are fewer people at the game', not 'there are less people at the game.'

Its - possessive. Only used in the possessive. Its own façade was cracking.
It's - a contraction of it is. 'It's a shame' for 'it is a shame.'

It's / its is purely usage. I have a mental block when it comes to these words. I know the usage, but I type something else.

It’s is an abbreviation for it is hence the apostrophe

Should be whom not who! Lol

https://blog.oxforddictionaries.com/2012/09/07/that-who-which/

Whom does that sort of thing? 😁😃😄

They like to annoy the pedantic people.

"That" can be used insteadd of "who".

English is a funny language.