Trading fees are a valid counterargument here, and while they are non-negligible (especially back when the "90% of day traders lose money" rule was established), I don't think they account for the full 40%.

For point 2, if there is an expected return of 0, then on average this should push the portfolio toward 50% chance of profitability.

It is the psychological factors combined with a non-random market that ensure most traders lock in losses (usually after riding them too long or not long enough).