I'll guess: money. Postgres is decades old and was designed when the internet was smaller. Doing a large, fundamental change like this requires an already experienced person (or maybe more than one) to devote a lot of time designing and implementing some solution. This time costs money. So some company must be willing to employ or pay some people to work full-time on this for months. Anyone qualified to work on this should be very expensive, so full costs to pay experts for months of their time would be in the ~$100-200k level. Much outside the donate-a-cup-of-coffee-each-month range, and outside of any small startup's budget, too.

This suggests that the various companies employing people to work on Postgres-related stuff (like Microsoft, perhaps due to their purchase of Citus) have more lucrative work they'd rather do instead of improve this at the design level.

This problem is now perhaps larger than open source is designed to handle because of how expensive it is to fix. Very few people (zero in this case) are willing to freely donate months of their life to improve a situation that will enrich other companies.

Regarding the difficulty of doing this: the blog post here describes how the concurrency and transaction safety model is related to connections, so any connection-related work must also be aware of transaction safety (very scary).