Why can't all items be bought with the Monet that you make.

The reason for this is really simple -- The Simpsons: Tapped Out is a game from a genre called the "Freemium Grinder" type of game.

Rather than charging players a fee for the game itself, the game is free. To generate Income the game contains a mixture of free Content and premium content. The content in Tapped Out that costs Money (Donuts) is NOT required content. In other words you can play the game just fine without it. If you want that content though you have to either spend real-world money or save up the Donuts you obtain through regular Game Play.

To put that in perspective for you, a player who has been playing the game from day one and who has not spent any of the Donuts that they earned since then should have something like 400 Donuts saved up. Basically that means that there are hardly any Items in the game you could not get if you really wanted it, and without spending Money.

Of course the reality is that you are going to want far more than 400 Donuts worth of Premium Items. You may not enjoy the fact that the premium Items and objects cost real world Money but keep in mind that the game -- the high quality fun game -- would not be possible without that source of revenue for EA.