You seem quite worked up, and seem to be having a discussion about how a motorcycle would perform in a collision with a large car versus a small car colliding with a large car. To be clear, that's not what I'm talking about here. I was picking up on the parent post's question about why a motorcycle should be road legal, and this thing not, when the collision danger was probably equivalent or higher.
I'm not saying that this car is more dangerous in a collision than a motorcycle. It might be, though. This weighs more than a motorcycle, which means more mass involved in the collision, which means energy to be dissipated. In a basic understanding of physics, this is sometimes referred to as the mass-energy equivalence. It's denoted by the formula E = mc2 which is fairly well known.
It's not just about mass though, is it? It's not even just about collision safety.
Cars have features like airbags, crush zones and impact protection cells to make collisions more survivable. Some of these features are optional, but many are mandated by governments on new cars.
One of the reasons this vehicle would not be road legal is because a passenger getting in to the vehicle, or someone buying it second hand, would reasonably expect it to have these features. This is what I mean by obvious and non-obvious dangers. A motorcycle has obvious dangers. It's clear to anyone that if a car t-bones you, that car will impact your body directly. This car, however, has the appearance of "a newish car". What that means to someone in a developed country like Australia is "a vehicle that meets basic safety standards". This vehicle contravenes that expectation in a non-obvious way.
Regulations also exist around things like handling dynamics in collision avoidance scenarios. How does this vehicle perform if you need to swerve suddenly to avoid a kid that has run out in the street? Will it turn or skid onwards? Will it roll over? Will something else happen? It might perform wonderfully, but the certifying authority doesn't have any reliable information that someone reliable has tested it.
New vehicles sold also have regulations around the reliability of individual components. The master cylinder for the braking system must be manufactured to a given standard of quality. If the plastic casting is crappy and the lid blows off when you hit the brakes, that's a bad thing and someone could be hurt. Preventing things like this happening to unsuspecting bystanders is why we live in a society that has agreed to place restrictions upon itself for the greater good.