Why do some people get offended when someone says “Oh my God”?

What’s wrong with that. I said “Oh my god” and some kid said, “Can you not say oh my god and say ‘Oh my Goodness’ instead? That’s offensive to God.’ His parents looked at me with disbelief. I was looking at a video on my phone minding my own business.
My sister-in-law HATES when people say “Oh my God” and prefers to say “Oh my Goodness”.
It is fine if she prefers it, but it’s annoying hearing that a lot.
What is wrong with saying “Oh my god”?