Because it's literally world shifting. I'm sure they'll reach the point you're saying at some point but they can't go from "Oh dad's not my dad" to "but he raised me so he is" in .2 seconds. Hell it took me a couple days to come to terms with the fact that I have an aunt my dad never told us about and that my mom knew about it. When she reached out to my sister I thought it was a scam until my mom confirmed she was who she said she was. The OP has to not only process this new information but process how this changes literally every interaction they've had with dad's best friend and if he knew they were his. It also drastically changes their medical history.

It's also possible (though unlikely) that they were having trouble conceiving and they decided to use him as a sperm donor and were successful. Or that the mom and his best friend were together before, they broke up and the non-bio dad decided he loved her and wanted to raise the kid with her. Or the mom is just a cheating scumbag and the husband never knew. So many possibilities.