Well you could just built in the charger into the car

And just pull out the wire to plug into an outlet like those irons with retractable cords

But now your stuck with one type of outlet

You have 120v which is the one you plug in your phone to charge

Then you have the 240v that dryers plug in

Then you got the various different charging standards at charge stations that do the fast charging in different way

Like CCS and CHAdemo and of course Tesla

So for the sake of portability, modularity, and cost

We keep the charging on the outside