Why is real-money-betting necessary for poker to be fun?
When you fall behind in a "for fun" game, the best strategy becomes to make high risk yet high reward choices. Essentially, if you are sitting in last place, you've got nothing to lose so you might as well risk it all for a shot at the top. 

In real life poker, you are always in a situation where you have something to lose: your money. Every dollar counts, so players will not change their strategy.