Quote from 1a2b3cppp:
Three main reasons:
1) there's no emotional involvement in demo trading. Who cares if you lose money? People get nervous when trading real money.
2) some demos don't use realistic fills, so even if you do everything mechanically indentical in a live account, you get worse fills = potentially lose money.
3) people will backtest their system and curve fit it like crazy and then it won't be profitable going forward. Live forward testing is a good idea to help avoid this.
But the main reason most traders lose money is because they don't have a profitable methodology. A lot of people say "it's the trader, not the system." They are wrong. It needs to be both. You have to have a profitable system and then you have to be able to follow its rules.