I don't know much about the web design of online casinos since I've never been on one of the sites, but traditional casinos are designed to make it slightly physically difficult for people with gambling problems to walk away, even if they think they want to. They're labyrinthine, lack landmarks, and don't have huge signs pointing to exits or places to cash out. If you walk in any one direction, you'll usually find at least one game that catches your interest before you leave.
I would assume online casinos don't flash a big button for you to cash out right after a big win. On a personal note, I've seen enough videos of online blackjack dealers cheating to never want to even give these scummy sites a visit, so I can't say for sure. Online gambling is much worse than brick and mortar. You can do it at home in your underwear so it is much easier to tell yourself you are only going to play for a few minutes between chores. It is also much easier to hide from loved ones than going to a casino for hours at a time. The games are much faster online than they are in real life. Finally, pulling out a credit card to buy credits on a computer screen is much easier mentally than pulling out cash at an atm and then plunking bills down on a table for physical chips. The discipline can be hard. And there are times I move my goal, but those are usually situations where I'm up a lot early in the trip. For instance, make 8pm dinner reservations before hitting the tables at 6pm (I do this a lot because it gives me a hard stop). If up a significant amount (like >50%) after dinner then only gamble with a portion of my winnings with the aim of double or nothing. So if I was up $500 after dinner, hit the tables with $250 with the aim of ending the night up in the $250-750 range. The tough part of discipline is setting your limits and sticking to them thru the highs and lows. Don't chase losses and don't get giddy on the highs.