The limiting factor for electric vehicle production is batteries. Trailers spend a lot more time sitting around than tractor units. It's more economically efficient to use the batteries as much as possible rather than have them sitting around.

Trailers come in many form factors: container, flat bet, end-dump. Heck, sometimes you just want to tow another tractor. Those different form factors have different constraints in terms of battery size and shape.

You'd still need big batteries in some tractor units so that they could drop off trailers. All tractor units would need small batteries so they could go short distances without a trailer.