It also requires much heavier and "non-standard" coupling between tractor and trailer since the trailer now needs to power the tractor rather than the reverse (and needs send significantly more power over the link)
Weight is also no problem as 500kg of batteries is nothing compared to 20 tones of a trailer.
There are power cables but they're used to power the trailer from the tractor rather than the other way around, they're mostly controlling trailer devices (lamps, brakes, etc…) and the general power ones are small potatoes to power cooling (refrigerated trailers) or lifts, not full-blown truck engines.
> Thing is to just make a thicker one.
Well yeah as I noted with that "solution" you need much heavier coupling than the standard ones (and reversed power), so you get to throw all existing tractors and trailers in the trash, rather than just replacing the tractors and keeping all existing trailers.
And that means much larger upfront costs for debatable benefits.