as its said in the book "theory of fun" games are all about learning. a game is still fun that still teaches new things to the player. but the problem is I cant really understand concept of learning in many games. for example what is learned in a shooter game? what is learned in a casual phone game that is all about smashing zombies and has abut a million downloads?

is really  learning most important aspect of great games? 

What's learned in a shooter game is how enemies behave, how to use the environment to avoid or conquer enemies, how to use the game's weapons. What's learned in a casual mobile zombie smasher is how enemies behave, how to use the environment to avoid or conquer enemies, how to use the game's weapons. Try it. Play a game. Try not to learn anything. Then think back and realize that you did learn something. 

Have you read Daniel Cook's Chemistry of Game Design?

It talks a bit about players being exposed to skills, getting feedback from the game on how those skills interact with the game's world, and then eventually mastering those skills through practice.

This sort of learning is present in every game.

Or the reverse, why did you stop playing an old game?

The game itself hasn't changed (it has fixed rules), so what is it that you find lacking there?

I think, to be fair, that 'learning' is not really the best word here.

Ultimately, it is about developing and progressing a skill(s) (which in a game could be more of a reflex then a deliberate cognitive/conscious ability), or at least feeling that that is the case.

Which is 'a kind of learning, more or less'.

Also, the more I think about this, the more I find reasons to disagree with it...

While 'learning' seems to be important to the longevity of a game - in that it enables you to get something new out of later players - it doesn't seem a completely necessary component. Otherwise games like candy crush or snakes and ladders wouldn't have any appeal. Same with math games, word searches and so on.

I think, really, there are only two completely necessary components to games:

An active participant.

A series of events and/or final outcome with at least a minimal amount of uncertainty.

I think that covers all understandings of the term 'game'. Everything else is more a measure of how good the game is.

6 hours ago, SomeoneRichards said:

'learning' ... doesn't seem a completely necessary component [of games]

It's not "necessary" - it's unavoidable. When designing a game for entertainment, learning is not necessarily something a designer builds in. But learning is an outcome of playing games. Heck, it's also an outcome of making them!