Skill games are hard to realize cause it ends up in who is skilled in cheating and writing the best bot. In p2p games the majority of fair players will lose interest fast and when betting against the house you have to either increase the difficulty + lower the player profits (which will annoy fair players) or work hard on anti-cheating measures and trying to prove people are indeed cheating.
I disagree. There are plenty of skill games, that are extremely difficult to write a bot for. Even something like a rock-paper-scissors bot is a daunting challenge. Deviate from the nash equilibrium (a random throw), and risk being exploited. But if you play randomly, and you have only downside (assuming a rake).
I think in a game like that, there would be virtually no bots -- and a lot of players.
But yeah, there's definitely whole classes of games which aren't really suitable due to bots (e.g reflex based, and many strategy based ones)