Chrome OS and Android are based on the Linux kernel. Hence they are Linux operating system.
But at the same time, "we" often do not count them in same family as CentOS, Fedora, RHEL, Debian, Mint, Ubuntu touch, Mobian and so on.
But how come? How do we distinguish between Chrome and Android vs the other; how is the separation defined? Or do you disagree with the starting point?
I think the main thing is point 1 that @phi1997 wrote.
I think basically ChromeOS and Android are things created by Google for the purpose of being good for Google, in mane ways exploiting users in the process. Google's interests are opposed to the interests of users who want to be free. Google wants control.
In contrast, the "real" GNU/Linux distros are following FOSS ideals, meaning that they are developed based on what is good for users.
1/2
The main point of FOSS is that the user is in control and therefore the user will not be abused by software developers. FOSS is meant to bridge the gap between users and developers, so that developers no longer have power over users.
Google is essentially abusing FOSS by creating things that are "open" but still in practice controlled by Google, so Google is in the end able to abuse users in similar ways as is usually done through proprietary software.
2/2
Quote from https://www.gnu.org/philosophy/free-software-even-more-important.html
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If the users don't control the program, the program controls the users. With proprietary software, there is always some entity, the developer or “owner” of the program, that controls the program—and through it, exercises power over its users. A nonfree program is a yoke, an instrument of unjust power.
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Google is exercising such unjust power also via "free" things like Android. Google gets away with that because Google is so big.
*technically* Linux, yes, but in practice people will easily misunderstand, it is confusing unfortunately.
You can try to write something else to clarify what you mean, like how @linmob writes "GNU-like #MobileLinux", see https://fosstodon.org/@linmob/109893712566955547
Note the "GNU-like" there, that's because it's not always GNU, like postmarketOS is not GNU, instead based on Alpine Linux. While PureOS, Mobian, Manjaro etc are GNU/Linux distribtions. 😅
@hehemrin Couple big reasons I can think of for why they're seen as separate:
1. ChromeOS and Android are way more locked down
2. Accessories and games may be marketed as for ChromeOS or Android, but if something's meant for use with any other distro, they'll usually just say it's for Linux (Or, occasionally, for Ubuntu)
3. Google doesn't want Linux's reputation of being mainly for nerds associated with Android or ChromeOS, so they don't mention it much in marketing