Why not put it next to the camera? That’s what my $200 Pinephone CE does (and the $150 model with the same sensor), and it works well IME.
Unfortunately, it boils down to org charts and who talks to whom in a company.
An engineer who owns the proximity sensor device driver is usually not invited to the meeting where the industrial design lead is showing off the latest iteration to the product and marketing team.
My personal opinion: most obvious flaws in consumer hardware is just a reflection of flaws within the OEM’s org chart and the networks of people within the chart.