why angular 2 or 4 doesn't have popup window or modal window and we have to implement it by ourselves? Is it because single page application by design shouldn't have popup windows or is it because it would be hard to maintain popup windows for the mobile segment?
I'm guessing Angular didn't include them because they didn't include any ui elements. It has no input box or drop down or any visual elements. It depends on html, material design, or other libraries to provide them.
You can find the modal in material design here: https://material.angular.io/components/dialog/overview
Personally, I think modal dialogs take away from a good user experience and they should not be used if there is any possible alternative. But this is just my opinion.