The three point harness is a good compromise between the uselessness of a lap belt and the incovinience of a 5 point harness. But its certainly not as good as a 5. Point harness.
3-point belts are made to be as quick and fool proof as possible so that people actually use them, and use them correctly. Importantly, they auto-retract so that they are worn in the proper position.
When you put on 5-point belts, you have to 4 times as many latches to operate, and you have to adjust all 5 belts -- and in the proper order. If you tighten the belts in the wrong order, they won't protect you properly. It is not intuitive -- I have a car with 5-points and the passengers who manage to figure out how to latch them almost universally tighten the belts wrong.
Edit to answer my own question: apparently they use spotters that radio in to them whether or not it's clear, and (at least in NASCAR) the cars don't have side mirrors but have an extra long rear view mirror.
When I was a kid on the farm, all the transmissions in the grain trucks were modified so you could only go forward. Too many accidents driving backwards with bad sight lines. Its a strange thing at first but you get used to always parking with a forward exit.
Consider for instance the $34 million verdict in Texas against Dorel, which failed to warn against placing children under the age of two in forward-facing seats. The kid Cayden was properly restrained in the back seat, but the weight of his head alone was enough to result in spinal injuries that have left him partially paralyzed for life.
We used to laugh at my kid when kid tried to reach for things on the floor and then fell on their head due to the weight of the head relative to the rest of the body -- but that's why rear-facing and five-point is important for children under 2,3 years old.
Go look at crash tests of "child car seats" that are nothing more than a box ticking exercise which uses the adult seat, versus a properly engineering seat and you'll see what you are completely wrong.
https://www.cdc.gov/motorvehiclesafety/child_passenger_safet....
But second, that doesn't matter. Most places do not have laws that prohibit children in the front seat of cars, only that they use appropriate child seats. Further, child seats are typically required for _children_, not just infants. IIRC, for example, in Texas, child seats are required for any child under the age of 8, unless they are taller than 4'9". It is totally reasonable for a 7 year old to want to go for a ride in their parents "fancy" car. It's actually very responsible of this person to get a child seat properly installed in order to support that.