There's a one on one relation between a page and an object at this moment.
It can be very helpful to create pages with fields from multiple objects on it.

At this moment I can only select fields from the object itself on my page editor, while it would be very nice to have at least the fields from related objects available as well.

As a workaround now, you'd need to use client-side scripting or create an additional object with a bunch of formula fields to accomplish this. From a technical point of view, it might be easier to do this only for objects that have relationships (otherwise it would be hard to know which data to show from which record).