Why motorcycles arenot efficient?

I drove my diesel car and the fuel consumption is about 5.5 liter per 100 kilometer (like 51.4mpg). It means I can drive my car, my family, my dog at any weather for 800km out of town and have no doubt about it.

The diesel VW Passat 2.0 Tdi consumes about 6.5 liter per 100 kilometer while driving in the city. This is a car that weights about 1500kg.

I remember ther was a time when I had used my Kawasaki Versys 650 for community riding and for the trips to the neighbors countries.

It was about 5.5 liters per 100km in the city and about 5.0 liters for the highway.

The difference is 0.5 liter per 100km between the 200kg bike with 100kg rider some baggage and 1500kg car, the family of four and the baggage.

I understood about highway driving and the air resistance but still do not understand about the huge difference Between a car weight compared to the bike and a small difference in the fuel consumption.

I had not drive the Versys very aggressively. The lane splitting is grey zone in Ukraine but I used it a lot because of hard traffic in the city.