Owners of car companies, to make more money. More disposable, more expensive cars, in less easily entered industry. How else will they keep BYD and others from coming in?
How does this square with your theory that cars are becoming 'more disposable'? They seem to be running longer than ever before.
Repairability becomes somewhat less relevant when reliability is better out-of-the-box.
[1] A decade ago it was 11 years.
Also, unlike with money and wealth and other metrics where averages aren't very useful, the distribution of car ages does not have a tail of incredible outliers. There aren't a lot of billion-year-old cars driving that average away from the median.
Look, it's entirely possible that 'this time it'll be different', and we'll regress on this metric, but at the moment the data does not support it.