Because screens with different sizes and requirements require production lines and specific technologies (as well as a variety of different manufacturers of OLED panels from which to choose the best offers, which currently does not exist …).
For now, therefore, Apple has used the excellent OLED screens of Samsung only in the iPhone X, while for the iPad has adopted the excellent IGZO screens of Sharp. MacBooks Pro also use top-of-the-range screens.
Not even all iPhone models for now use / will use OLED: Who produces Apple iPhone screens?
In the future, when more manufacturers are able to produce OLEDs with the same characteristics as the current LCD screens and with adequate size, Apple could switch from LCD to OLED across the range.
So until such time that the production process will be easier for them, then Apple Will Be Using OLED on iPad, MacBook and iMacs.
Now regarding the battery not replaceable?
Certainly to create a product with a lighter design.
However, replacing an iPhone battery is much less complicated than one might think. On the market, you can find different kits , like those of the German company Giga Fixxoo , which allow you to replace a malfunctioning battery. Online there are also excellent tutorials on how to use these kits (they also have screen repair kits ).