I think many "mistakes" are intentional, although perhaps not by both parties. I know one situation where a guy was afraid he would lose his girlfriend when she went to college. They'd been together for years so she completely trusted him, and in this case trusted him with birth control (i.e. pull out). He betrayed that trust out of fear of losing her. She was there one week when she found out she was pregnant. She dropped out and they married instead. Isn't that sweet? /sarcasm
Many young immature women think a baby will complete a void in their life. Perhaps some are reckless and irresponsible men generally don't have to worry about what happens with casual encounters.
All that put together adds up to many babies that are born without secure, loving homes.