Disclaimer: I have my thoughts, but they are just my personal thoughts. I do not participate in these fetishes and I’m not close to anyone who does—therefore I am no kind of authority.

That said—I think that if it were socially acceptable and normal for CAMAB folks and men to wear clothing styles and makeup marketed toward women or engage in “feminine” activities, there would be fewer people who fetishize “forced feminization”. Our culture views it as humiliating and taboo for CAMAB folks and men to “degrade” themselves by being feminine. This says volumes about how patriarchal, misogynist, transphobic, and femphobic our society is. If being feminine were really valued equally as being masculine, there would be no aspect of degradation in this kind of play. 

For some who engage in what’s called “sissification”, though, they simply like feeling vulnerable and/or pretty. I personally feel uncomfortable labeling this desire “sissy”, but that’s the language some folks use.