They don't realize how much money they've spent trying to recoup their losses - they only realize it when their bankroll is low and they're facing financial problems.
I've never counted my losses so far - that's because I only play for personal, temporary enjoyment. I don't spend much money on experience - sometimes it's just an attempt to understand how the game appeals to me. I consider winning and losing normal - nothing more, but there are times when the urge to gamble comes when days are boring.
I think that if gambling platforms displayed the results of wins and losses over time for each user, similar to how exchanges do with futures, it would be useful for some gamblers. At least a user would be able to clearly see how much money he has lost over time, and that it's time to stop.