They say that sports betting has no house edge, but why is it it's still hard to win here?
You are mistaken my friend. Sports betting have higher house edge compared to online crypto-based dice and some other online crypto-based casino betting sites.
Take a look at this English Open in snooker moneyline. Both David Grace and Michael Georgiou are 50/50 according to betting experts. But if you bet on any of them you won't get 100% return. Instead it is only 88.87% meaning the house edge is 11.13%. That is much bigger than the 1% to 2% house edge on most online crypto casinos. The only difference with sports betting is you cannot purely rely on luck. You can study and assess the players and their past performances. The only games that aren't dependent on luck on casino games are poker and other games that you can play against other players.
I mentioned "online crypto-based casino" because land based casino have higher house edge, probably 10% to 20%. The same with online fiat-based casinos where house edge are probably the same with land-based casinos.
My friend you cannot expect to have regular income on gambling. It is a game of chance, luck and some needed analysis. You better gamble responsibly just for fun and do not have high expectations to profit.