South Korean company tried to do just that - they built an OS called "Tizen" which withered and died because without user-facing software, it was a horrible user experience and people were seriously annoyed that they couldn't use the software they actually care about. Instead, they went and bought Google/Apple devices instead.
Another USA company tried to do that as well. They called their OS "Windows Phone" and it withered and died because a fractured support for software people care about doesn't make a useful device.
A Chinese company tried to do that as well. They now again ship Google Services next to their own Huawei services in Europe because without software support, their smart devices weren't useful to users either.
Where are you getting your information? Tizen OS is not dead. It is well alive and kicking and being improved and Samsung is slowly implementing that unified OS not only for their phones, but for other Samsung products. The main issue for any phone producer is that any OS that they do not own also Android comes at a price - there is no such thing as free OS.
There are also plenty of other mobile phone OS, that are not compatible with every phone and that is the main issue why they are not widespread. Phone that I own is supported not only by Android(and Android forks), but also couple of other mobile phone OS. The trend is that in future there will be more choices for OS, that might satisfy those users, that are currently not happy about Android, but have no other choices at the moment.
There are at least 10 other mobile OS choices - most of them are based on Linux, but current share of those is ~0,1% out of ~6 billion of phones. In total numbers that is only 6 million devices. 6 million device market is a significant number for any company, not to mention, that this number is only playground compared to 1000x larger world market of mobile phones.
The relationship is symbiotic. iPhones need your apps to stay relevant, and developers need access to the iPhone ecosystem to be able to reach anybody.
You're almost there, but you didn't go all the way.
South Korea is a small market. The US company is famous for mismanaging its mobile operating systems for more than 2 decades.
The Chinese company didn't "try" to do that, it was forced to. But guess what, in China, where most external apps are banned, there's an entire, completely separate ecosystem.
You do need a combination of skill and critical mass. I don't expect South Korea to achieve this, but for example if there's a bigger alliance of say, South Korea, the EU, etc., where they put resources into encouraging an OS based on open standards, that starts to look feasible.
App developers will need support, but they can port their apps. Especially since this OS doesn't need to start from scratch, it can be based on AOSP.
That's probably the biggest thing Google/Apple are afraid of this point, that a fully open/interoperable mobile OS is somehow enforced. They either have to open up their OSes to be in key markets or they're forced to let someone else create that OS.
South Korea has its own Galapagos Island situation with apps thanks to efforts to foster them by the government — eg Google Maps is seriously crippled and you have to use Kakao Maps, they have their own local version of Uber, etc, etc
Yeah, but with enough of these "nudges", maybe it finally happens.
The EU has already fined Google, South Korea is adding these new rules, I think even the US is thinking about some regulation, let's see what happens in a few years.
Another USA company tried to do that as well. They called their OS "Windows Phone" and it withered and died because a fractured support for software people care about doesn't make a useful device.
A Chinese company tried to do that as well. They now again ship Google Services next to their own Huawei services in Europe because without software support, their smart devices weren't useful to users either.