I'm not a store developer so can't comment authoritatively but I have an option about why so few are developing patterns, or comprehensive block solutions.

While patterns have been in core for nearly a year I've felt like if I develop for them they are still likely to change and break my bespoke element. So I feel cautious and have looked for stopgap solutions while I wait to see how it all plays out.

That might be a wrong assumption but that's my feeling from the project as a whole - it's constantly in flux. I'm not designing for thousands of theme owners I'm designing for single clients, but I must support any breaking changes - theme developers have to support thousands of users. That's a commitment.

so for my case I make sure that I build on stable static foundations. Nobody wants to admit this but while gutenberg has been introduced it has created FUD in many people's business processes, and that's potentially fatal to small teams.

So .. here's the really crazy part - I've felt so cautious for developing for Gutenberg that I've used Elementor as a prophylactic shim between myself and any breaking changes. I opted out of developing Gutenberg solutons for clients and I created modules for elementor instead. Now, we all know the pitfalls of elementor (slow, fat, etc) but as far as client support risk it's more practical to accept those pitfalls and develop for this "shim" layer.

It's an unpleasant solution, but I had to make the choice, Classic (legacy issues) / Gutenberg (Uncertainty anxiety) / Elementor (slow and fat). I chose the latter.

tl;dr I believe that small dev teams are not fully committing to Gutenberg/blocks/patterns because it feels like an ever-shifting and uncertain landscape and that uncertainty may cost us scarce human resources to support. Everyone is cautious and looking at options right now.

That's my assumption anyway. Probably way off.