They are made, they look like mini chainsaws and are commonly used for carving bowls.

A common brand for these blades is Lancelot
You can also get smaller saw blades in the 4.5" form factor. I suggest you go to a saw store or woodworking shop. Here is a product on Amazon:
http://www.amazon.com/KwikTool-BB450-2-Inch-1-Inch-8-Inch/dp/B000O3IOEW/ref=sr_1_2?ie=UTF8&qid=1357252670&sr=8-2&keywords=angle+grinder+wood+blade
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They are made, they look like mini chainsaws and are commonly used for carving bowls.

A common brand for these blades is Lancelot
You can also get smaller saw blades in the 4.5" form factor. I suggest you go to a saw store or woodworking shop. Here is a product on Amazon:
http://www.amazon.com/KwikTool-BB450-2-Inch-1-Inch-8-Inch/dp/B000O3IOEW/ref=sr_1_2?ie=UTF8&qid=1357252670&sr=8-2&keywords=angle+grinder+wood+blade
The primary reason is grinders typically spin at 10,000 rpm. Wood cutting saws typically spin in the 3500 to 4000 rpm. If you spun a typical circular saw blade at 10,000 rpm it would come apart. As @Matthew PK has shown they make wheels for working wood with a grinder but they are not the typical toothed blades.
Recently learned that they sell mini circular saw blades that fit into a 4.5 inch angle grinder. That would be pretty damn convenient for dealing with small trees and such in the yard, but it seems like an amputation waiting to happen. Are they safe when used right? Seems like it would be prone to binding and kicking.