why don't other beginners castle?

I noticed that a lot of my opponents don't castle even though it's one of the first principles I learnt in chess. I'm wondering why don't people bother? Is it just that they don't want to use up a move or a lot of checkmating study revolves around how to checkmate a castled king?

Maybe they don't know how on the computer.

For the most part I think this is why. I can say though that I've gotten into a lot worse positions when I don't castle in good time. Castling early is good. Not castling early is a taking a big risk. It's a lot easier to take advantage of an un-castled king.

Same here. Happened enough times for me to know that I should do it when the opportunity arises. 

Either they are not aware of castling or they do not know how to execute castling on this site. Or perhaps they prefer to move queens around because they think that launching their queen means "Here's my queen, and all your pieces shall be captured henceforth" and do not bother castling as a result.

Some elementary checkmates (like back rank mate) usually happen against a castled king. 

I've heard more than one beginner say they think castling is bad because it puts the king in the corner making it easier to checkmate... of course in the great majority of positions the opposite is true  (kings in the center are much easier to attack) but since they're beginners they don't know this.