Fair, but this is getting fixed. Postgres 16 (which was just released) adds an origin tag to logical replication, so that you can know whether or not to pass on a change (and avoid having an infinite loop where a change is replicated back and forth). That's the first step to having multiple mains. I think in theory, if you had deep Postgres knowledge, you could set up a high touch, bespoke multi-main deployment today.
I expect the replication story with Postgres will get much better over the next few years.
The writes aren't. There's no sharding for example. You can ever only have 1 primary writer.