Because it took 10, 20 years for them to start to know their ass from a hole in the ground. So they take all the pain of their learning experience and lob it back at you whenever you remind them of themselves starting out.

They might also resent newbies for the much better learning materials available today and even the possibility for easy shortcuts (llms). Back then there was no substitute for sitting down and fiddling with it for hours or reading a some poorly-written book.