Storage and production costs for something available in UHD are the same regardless of whether someone streams a downscaled version. Storing multiple resolutions (assuming they do that rather than downscaling live from the stored highest-res or making the client do it) is where additional costs for more versions would show up.

In my view, the "native" format that they produced the content in is the format they should be delivering by default (if supported by the viewer's setup). Of course I recognize that people will disagree, after all, UHD Blu-rays also generally cost more than 1080p Blu-rays. But of course, UHD Blu-rays aren't backwards compatible with standard Blu-ray equipment, so it doesn't feel like an artificial limitation simply for the purpose of price discrimination. This also seems to be a no-brainer for Amazon Prime, Apple TV+, and Disney+, which (as far as I can tell) don't have any concept of charging based on video/audio quality.