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

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.'