It's not that easy to get a girl pregnant. I eventually did but I had came inside of atleast 70-80 girls, most who I fucked a bunch of times, before that happened.
Unless they are not on birth control and also ovulating, chances are slim they'll get pregnant. Pretty much every girl is on birth control these days so yeah...
Also, you are assuming these girls are being up front if they do get pregnant. Plenty of girls don't tell the dude.. Good chance you've already impregnated a girl and just don't know she aborted. This is especially more likely to happen if you are just having casual hookups rather than relationships.