Why is there pressure to get married sooner rather than later is a different question from why get married at all.
If you accept the premise that you should get married at some point, the pressure to do it now probably comes from the sometimes correct and other times mistaken that doing it earlier will result in a better marriage. That’s sometimes correct, as I think it is easier for younger people less set in their ways to fuse together and build a household together. If one of both is not ready though, it is a recipie for disaster.
So to answer the question why is there so much pressure from your parents to get married, it is because that is what parents do, have always done and will always do.