because guys that usually move in with women tend to be needy , not as attractive , or socially inept.
men who are in demand or have a very high smv dont move in because they live life like women , getting hit on and finding sex and relationships easily, who would want to give away all that power? on the flipside some high value men do get girlfriends and move in and have successful relationships , but its usually cause the woman feels lucky and feels like she has a catch , which keeps her in check and on best behavior.
women will sleep with lower value men but will not commit or be faithful or sexually monogamous with lower value men, or men who arent their ''first choice''
the hard part is knowing for sure that you are her ''first choice'' and then remaining that