Natively supported means part of the core product.
Dealing with third party solutions means having to worry about incompatibility, enterprise support, ongoing maintenance etc.
Not being in part of core mean multiple solutions to fit different needs. Also a lot of people don't really need HA so it keep core feature more focused. (Again HA is supported at low level, you could totally achieve HA without third party, it just require more internal ressources)
There are a lot of companies out there which provide enterprise support for PostgreSQL + HA.
An actively developed FOSS solution backed by EnterpriseDB also one of the main company supporting PostgreSQL.
But err... And ahem ahem... there are more https://www.postgresql.org/download/products/3-clusteringrep...
It's a choice of the PostgreSQL development team to only include low level api for HA in core project. But it's fairly harsh to say there is no active support for HA. Many maintainers of the core are also invested in HA related project build around PostgreSQL.