Why No Cargo Motorcycles?

Here's a thought that's been ruminating in my head for the past few days...

We're all pretty familiar with the various cargo bike types out there: Bakfiets/Long Johns, Long Tails/Xtracycles, Porteurs, Butcher Bikes, and even various types of Trikes (Rickshaws, Tadpoles).

Why aren't there motorcycle or scooter equivalents? Our two-wheeled brethren are utilized by many folks as practical urban runabouts, but one of the biggest limitations of motorcycles and scooters seems to be that they can't easily carry the day's grocery shopping or a piece of luggage. I've seen many a scooter-rider precariously balancing a couple shopping bags between their legs on the footboards, or "delivery scooters" with a huge box mounted at the back where a second passenger would otherwise sit -- that seems like it'd create a too-high/rearward center of gravity and ruin the vehicle's handling.

I'm envisioning something like a Bakfiets/Long John front-end mated to a scooter's rear end, which might result in a very practical urban runabout with useful cargo capacity.

Why doesn't something like this exist already?