That's the magic phrase that really does the heavy lifting here.
Companies and leadership have to define what "good enough" is for them, and in almost every case that comes down to profit and prestige (or power if you take it a step further).
As you said, developers are costly. When a company just wants profit it usually makes sense to hire someone just skilled enough to make the product shippable.
I believe the flow state is why it's so easy to believe the idea of the 10x engineer, we've all experienced the feeling of flying through code without barriers, and we have all felt the below average days where you may be tired and just can't quite load all that complexity into your head to get any real work done. So we believe that there are people out there who are so skilled that they exist in this flow state all the time.
Yes, developers should work on themselves to rely less on a state of flow to be high performers, but even so there will always a benefit to being "in the zone".