Hi zubir,

That is because its server centric.

1.The constructor methods on the associated custom controller or controller extension classes are called, instantiating the controller objects.
2.If the page contains any custom components, they are created and the constructor methods on any associated custom controllers or controller extensions are executed. If attributes are set on the custom component using expressions, the expressions are evaluated after the constructors are evaluated.
2.The page then executes any assignTo attributes on any custom components on the page. After the assignTo methods are executed, expressions are evaluated, the action attribute on the <apex:page> component is evaluated, and all other method calls, such as getting or setting a property value, are made.
3.If the page contains an <apex:form> component, all of the information necessary to maintain the state of the database between page requests is saved as an encrypted view state. The view state is updated whenever the page is updated.
4.The resulting HTML is sent to the browser. If there are any client-side technologies on the page, such as JavaScript, the browser executes them.

Plase go through below links

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_controller_get_request.htm
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_controller_lifecycle_example.htm

Thanks.