[orwin]

Yes, this is only because american sports give "line bet" and "prop bets", that anybody can exploit if two apps aren't coordinated. You can do line arbitrage (middling is the easiest, but professionals use props with weird maths and specific knowledge to make a living).

If you only offer to bet on the money line, it makes it way harder for professional gamblers.

[blitzar]

A football game in the UK for example has ~60 markets on exchange for a single game with bets on everything from the result, handicaped result (multiple lines), the number of goals, the times of the goals, scorers of goals, half-time results, yellow cards, red cards, number of corners, number of shots at goal, time of first thrown in etc.