Is it allowed for women to engage in intercourse with male slaves like it is for men with their females. If not then why?

No it is not allowed. If she wants to, she must marry her male slave and she can marry only one.

It’s the same reason why a woman cannot marry more than one man at one time. One possibility wisdom behind it is determining who the father is in case or pregnancy. If she can have children from numerous slaves the father normally cannot be determined. You may say: today with DNA testing it’s possible. Well, historically it wasn’t, and even today most people around the world don’t have access to such testing or don’t have the money to get it done. Islamic laws are that would apply to most people in general.