It's not about not knowing about an optimization. The challenge is to know when to apply it, so that it does not cause regressions for cases that can't benefit from it. It may be less risky in specialized systems, like BI systems typically don't need to worry about regressing OLTP workloads. Postgres absolutely needs to be careful of that.

I believe that's one of the reasons why it took about ~8 years (the original patch was proposed in 2017).