I'm far from an expert on this but my understanding is similar. I think there are several technical and market reasons for this. First is the simple cost of developing and certifying a new general aviation engine. That cost must be covered by sales in an already cost difficult market. The second part is technical...plane engines are used very differently than car engines. They are not purely steady state but they are typically used in a near steady state way. This makes improvements such as fuel injection less additive beneficial. Ad in that most of the uses of the engines are non-commercial and non-life critical and you have a logic - why would I spend tones more to gain a few points of efficiency on something I use for fun that already costs me an arm and a leg that works. Its the same reason you don't see the most fuel efficient innovations start in sports cars.

example: look at the Lycoming O-360 and the list of planes it is used in.

https://en.wikipedia.org/wiki/Lycoming_O-360