Why do black people say “you heard me” after every sentence? Do people not pay enough attention to them so they need to check if they hear..?

same reason they "axe" you something.

OR they axe, "You know what I'm sayin?" after every sentence

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They probably have a hearing problem...................

The answer is known, and it is not in your so called "question".

Most blacks are slow.

That's why.

I've heard the phrase "you know what I'm sayin'?" often from black people. People in Japan do something similar if they think you really aren't paying attention: "moshi-moshi" (literally "hello, hello" but it might be better translated as the slang "Earth to [fill in name]").

ive only heard that line a few times in my life and its at times where i honestly could not hear them

You don't spend a lot of time in mixed race scenarios do you? Generally, "I hear you" is used when any subject matter that some else agrees with. "The Pope is Catholic", "I hear you" When they ask if you hear them, they are seeking confirmation. Also common is the "You feelin' me"? It's got nothing to do with their hands.

It's probably a cultural habit from the time when no one wanted to hear what a black person had to say. The affects of racism doesn't simply disappear.