iRacing using a simplistic system, it doesn't mean you HAVE to have it that way.
It just means that it'll take time for PD to fine tune the system they're trying to put in place which doesn't penalize players who were at 0 fault.
iRacing is also a subscription based game, which almost immediately caters to a different type of driver, being on PC means it's mostly wheel setups whereas GT:Sport more than likely is trying to tap into a more casual market.
A casual racer who might get a race or two in every day isn't going to be very pleased if an accident he/she is totally not at fault for is punished for it.
And while yes I agree a zero fault system in the end will eventually get everyone to the correct ranking, it will take the most time.
Think of this from a mathematical point of view. Drivers have an SR, which essentially makes them numbers, these numbers are shifting right now, but in the long run they will eventually even out.
PD's job is basically figuring out how to sort these numbers using an algorithm.
iRacing uses the slowest algorithm, it's tried and true and really doesn't have much thought put into it, but it works.
PD is attempting to create a faster algorithm, something they obviously have't mastered yet, but with continued work I highly expect they could figure out.
Your suggesting to simply use the simple algorithm iRacing has developed because it works. This isn't how innovation works, Gran Turismo Sport is trying to be the next best racing sim, not a recreation of iRacing. I fully expect that the SR system will be updated constantly and will eventually be a lot more reliable than it is now, give it some time.
Here is a visual example.
Essentially PD is trying to develop something similar to quick sort, a faster way.
iRacing is using dated technology and we don't have to settle for it.