Because I'm a normal person, and I assume my sons who I am attempting to raise right will be normal people and meet reasonably normal partners? And to assume that if they are heterosexual that all women are not threatened by their MILs?
I am so sick of the wringing by people who have 20+ years left before becoming MILs and their "fear" that they will be shut out. If you are already worried your daughter in law will take your itty bitty boy away from you before he even meets his first girlfriend you are that mother in law. You will be the mother in law who gets pushed out. It will be your fault though.
MILs are not always at fault, the posts about MILs are there because no one posts about normal family relationships. They don't need to. They post about dysfunctional ones, asking for help or a place to rant.