Why do people STILL have problems with interracial relationships?

I think it largely comes down to culture. At least in some cases. My Dad is Filipino, and would likely be mad if I married someone who wasn't, because that person won't be able to relate to Filipino traditions, customs, culture- all of the things that make us part of who we are. To marry outside can be seen as a loss of culture, which is hard for the older generations who have ALREADY faced people trying to oppress their traditions and ways of life to take.
There are different types of "problems" with interracial relationships. Like a minority person who has faced discrimination and racism from a white person their whole life would be fairly justified in being worried about their kid then marrying someone who they see as the 'oppressor'. Especially older minorities, who likely faced that kind of oppression to the extreme- I have a Great Uncle who was held in a Japanese holding camp in WW2, DESPITE being born in America. He was told by white people who were JUST as American as him that he was not American based entirely on his race. So, of course, he would be weary of white people. He would raise his children to be weary of white people. His grandchildren would be raised to be weary of them. For many minority groups, it is a generational trauma based on the poor treatment to their race in the past, which makes them feel safer to just stay within their own race. Which is fair to an extent, since racism still exists today- I'm not even fully Asian, and I have had people pull their eyes into slits in my direction before.
And then there are those who simply don't like people from other races. It would be the same kind of education as those who were oppressed- a racist white person back in the 1950s hated minorities. So they teach their son to do the same. They son teaches his son to do the same. So on and so forth.
Sometimes it is based on racism or stereotypes. Sometimes fear from the oppression they have experienced. Sometimes a mix of the two, or soemthing else altogether.

I can see some people's logic for it but I don't agree with them. As you say people are people - it is really important not to lose sight of that.

I was brought up with everyone are our brothers and sisters I have pased that down to my children while they were growing up.