I think it's because in TF2 you get a super fun, deep game that you can play for years and have a ton of fun and not pay a penny. You only pay for "extras".

In FarmVille, when you use up your virtual money you can't play anymore. You can't plant anything, you can't harvest anything, you can't plow anything. You just have to wait a few hours until your plants grow.

So your choice is make an IAP or literally watch grass grow.

And even of you could grind through that and keep playing (say by mining gol laboriously)... TF2 is just a more complex, more interesting game. FarmVille and the like seem to exist only to get you to spend money compulsively.

Because TF2 has made a very specific effort to avoid "pay-to-win" gaming. Most of the expensive items in TF2 are largely cosmetic; all of the seriously game-changing items can be crafted, found, or obtained through unlocks.

Plus, TF2's stock weapons are generally the most versatile, so there are few if any game-changing items.