Probably because they can. They sit behind the computer, or phone or whatever device with no intention of ever meeting someone in person... So these people can be whomever they want to be, make things up to suit the situation. Of course, not everyone is like that......personally, I find people like that despicable.
Why do people pretend to be someone else on the Internet? Why can't they tell the truth instead of letting people live a lie about who they are?
Well, I know what you mean I had it happen to me many times and I like to know why people got live a lie about their self? I've ran into men online who pretended to be women and vice versa... females online acting like men... it makes no sense to me!
One reason I can think of is trolling. Not like fishing or to bully someone but more like on YouTube. You can lie about who you are or act extremely outrageous. I admit I do it sometimes. It is fun to troll people. Especially when they think you actually have a stake in your ridiculous arguments. (Also my reasons wont involve things like fishing or bulling there the more obscure reasons.).
Another reason might be because there transgender. Pretending to be the opposite sex online might be the only way they can express there feelings. Please comment I'd like to discus my reasons further.
Because the Internet allows them to be whoever they have always wanted to be somewhat like an ego trip booster. It really serves no other purpose than self-gratification and chances are most will be found out sooner or later.
The worst part of this that I see is in online dating where anyone can be whoever they want to be to impress others and it really is hard to tell the fakes from the real ones sometimes so trust your gut as to whether you believe them or not and worry about what you tell others on here.
That is why I prefer some sugar baby dating services like Secret Benefits. I always know what to expect from the people there and everything is always ok. Have you ever tried something like that for you? Maybe you will find it interesting as well. Good luck with dating ;)