I dunno if there's a partition. The SS might think it's safer if they can reach him if they need to. But it's a huge vehicle. I'm surprised that the wheel could be reached from the back.
https://www.caranddriver.com/news/a23397702/donald-trump-presidential-cadillac-limo-beast/
Maybe they put in a little toy wheel like Maggie Simpson has, but just never told Trump that it doesn't actually work. It's probably all sticky from the Hi-C that splurted out every time Trump squeezed the juice box too hard while pushing the straw in.