Others might call it "gate keeping", but that implies an intent that I don't think is really there. I think instead that the math professors think everyone else is like them, learn like they did.
I'm kind of dumb and so things as abstract as higher math have come to me with difficulty. I find I have to truly understand a thing before I can possibly expect to remember it or use it. And apparently I am not quick to understand things like differentiation, integration.
I think I only grasped matrix multiplication, linear algebra when I tried to write a flight simulator from scratch (well, from a book actually, but it had you start from scratch with your own Bresenham algorithm for line drawing, write your own polygon clipping code, face culling, etc.).
I have done pretty good (I must say) in helping my daughters get through high school math though since I do understand it (now). I think it helps that I probably approach teaching the concept as though it is to someone, like me, who is not going to make the mental leap to abstraction. I suspect many professional math teachers/majors don't suffer my shortcomings and don't teach from that vantage point.