It's not about the electric motors; they are great at towing. The difference is the energy stored onboard. An efficient vehicle - regardless of powertrain, but we notice this more in EVs because they are far more efficient - typically stores less energy because it requires less (and having more would make it less efficient). Dragging a trailer of x pounds uphill is going to use a fixed amount of energy (because physics!); that energy is a larger portion of the energy storage of an efficient vehicle than an inefficient one, and so will have an outsized effect on range. Dragging a trailer of x extra frontal area with a CD of y for z miles is similarly going to use a set amount of energy that is also a larger portion of a typical efficient vehicle's energy storage.
To tow farther, EVs just need more energy storage. As the price and weight of batteries come down, this will happen. Not everyone needs a 500-mile EV truck (my wife, for example, only has a 200-mile bladder), but it does make sense for those that tow. The 500-mile range is only for its (relatively) efficient non-tow mode; while towing it may get "only" 250 miles.
As ecarfan pointed out, aero is the largest factor affecting range on the highway while towing or not, and given that efficiency is lowered by the square of the speed, towing at lower speeds is FAR better. Towing at 73mph is a bad idea if you are concerned about range. Heck, I'd expect almost a 25% reduction over 60mph even if not towing. (See
HERE for an 8-year-old thread with some non-towing numbers on how speed and elevation affect efficiency. The absolute hit is exactly the same on gas cars; it's just harder to see the relative hit in the noise of how much of their energy storage routinely gets wasted).
BTW, my cousin in the Seattle area has an appointment to test-drive an R1T on Friday. He asked me to go along, but I can't make it. I have an R1S reservation (we have a trailer on order and so will be among those towing soon), but haven't yet heard anything from Rivian about events or test drives in this area.