There is a significant learning curve - it is hard to play winning poker without a serious time investment... learning and practicing, developing discipline, learning how to size bets effectively, developing an understanding of your opponents' tendencies and how to adjust to them.
Obviously this is just the beginning, there are so many poker concepts you must learn to be successful (some of which can only be learned by losing initially), but these are just a few examples of why it is so challenging to win.
Meanwhile poker can be extremely frustrating for new players who think they've got it all figured out, but in reality they are just scratching the surface. It is very common for people to think they're much better than they are, mainly because they aren't even aware of the things they don't know.
A good example:
There are people on here who post about watching hands for a day or a week and deciding that a site is not random because they lost or saw someone make quads four times. "I got dealt seven deuce off-suit three hands in a row, never playing there again!"
Well people's brains are hard-wired to see patterns even if they aren't really there (think Jesus in a slice of toast or shadows forming a face).
In reality there are 52! (factorial, 52*51*50*49 etc.) ways to arrange a deck of cards, which is a number with 68 zeroes, so they can watch hands play out for years and years and see the exact same combinations rarely if ever.
If it's all the same to you and you think only in the short-term, this still won't stop you from thinking it's not something you're doing wrong, it's something wrong with the cards.
Good luck!