Why can't we call future from future in Salesforce?

You cannot call another future method from a future method. As per Salesforce documentation,

You cannot call a method annotated with future from a method that also has the future annotation. Nor can you call a trigger from an annotated method that calls another annotated method.
If you do not have dependency between f1 & f2, you can call both methods one by one from original class.

You cannot normally call a future method in another future method, but if your salesforce license has the pilot feature enabled then you can call a future method from another future method.

To get the pilot feature you ll need to contact the salesforce where you can also double or triple some governor limits of asynchronous apex in your org.