Because games used to be "nintendo hard". They weren't long, they just had tricks to extend the play time. Things like limited lives and continues. Emenies that killed you by just walking into you. Purposely programmed with bad play control. Starting over at the beginning of the level when you die. Alot of old platformers like Castlevania, Super Mario Brothers, and Mega Man could be beat in less than an hour, but were played for months because of their "cheating" difficulty. Since you had to start over so much it made the games seem long.
I had the same problem when i played the demo a few months ago. To fix it start the game in windowed mode and it should work. Then set your full screen settings in the options. You don't have to start it windowed every time, just the first time. It should work normally after that.
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