There are plenty of fish in the sea. The problem is that most of them are, like, big mouth bass or sardines or mudsuckers or one-eyed halibut (which are creepy). I'm not a statistician but I'm fairly positive that there are relatively fewer of those tuna or marlin you were hoping to snag in the sea.
In addition to there being statistically fewer desirable fish, you might not have the right bait for the fish you want to attract. Hell, maybe tuna isn't even native to the sea you're casting your net in.
See, so while technically, there are plenty of fish in the sea, it's still a stupid saying.
More to come on this topic...
Hey Beautiful, what's your name?