It comes from childhood ....
There's a few reasons ...
Some men seek out mature women because they want to be taken care of. It's typically about the mental and emotional support that an older woman can give to her man. There is also “mommy syndrome”, where a man needs a woman to do everything for him (cooking, cleaning, laundry/picking clothes) ... Other times it's from finding her life experience to be potentially intriguing; he wants to learn about things through her he feels he may have missed when he was younger. Who better than a "mommy" to teach these things? (Mommys [and typically female teachers] mold the majority of what we learn and experience growing up) ...
Does that give you some ideas as to why?