Why is it the same thing when dating?

The same thing over and over again when flirting/trying to hook up. The same thing happens way to much I have gone through 10 girls this year (sounds fun huh) and the same thing occurs. I flirt they flirt back then after 2 weeks they could careless or say they have boyfriends. This would be dishearting if I was looking for a GF but its even more dishearting when all I'm looking for is FWB's or maybe a realationship if it so happens. This happens all the time with me what's going on? What am I doing wrong I'm rather much sick of the high volume of girls I can only attract for a 2 week period