Why they don't use modern cities,fantastic worlds,alternative realities,people,animals and etc?

Because forests consist of vegetation, which is hard to model and render realistically. Plus they're trying to brag about their awesome lightning with all the god rays and real-time shadows, which is easily demonstrated in an environment with a lot of moving objects (leaves).

Ehh... dunno.

Then why post?

because im thinking on it, and got something.

It shows texture of the engine.

Because that's what the CryEngine does best. Notice whenever Crysis screenshots come up in a graphics argument, it's always shots of vegetation, forests, oceans, etc. That's because when you look at character models, Crysis doesn't look all that fantastic.

cause the Forest do Cry when ever you cut down a tree.
THINK OF THE DAMN FOREST!

Because they're the hardest things to render

This. Rendering a forest is the most intensive graphical performance an engine can do. The more organic and round in nature the harder it is. A city on the other hand is man made and very geometrical. Making city scapes dead easy. So when someone shows a fantastic picture of a city from one game and a fantastic picture of a forest that looks alive. You can bet that the forest engine is doing far more work to achieve it. Heck even half as fantastic and it's still doing more work.

In the newer engines the fauna slightly move. When moving through fauna it needs to interact, be soft in essence. While in a city, buildings don't need to sway with weather. Your character won't be walking through soft benches. Throwing in a flag or cloth here and there is nothing compared to making everything soft and needing it to interact with the physics engine.

The character models in Crysis are actually really really good.