Yep, that is a fairly common misunderstanding that the general public makes (and ends up with: if I just take the opposite decision then I will make money).

The main reason why 90% lose money is costs. That is it. Most people probably are optimised for losing money but the main issue is really costs/overtrading.

But related to this, most people believe that edge on profitable trades is very large...but in most markets, institutional-grade costs will still be a big chunk of your edge i.e. costs matter hugely.