Ublock Origin/uMatrix on mobile significantly increase website performance and battery life on the average site, by blocking a a lot of advertising and tracking content.

Potential problems could be mitigated pretty easily by warning about or throttling extensions that use too much CPU/memory on mobile.

Not allowing extensions on Chrome for Android might have been an acceptable excuse in the early days of Android, with very weak single core CPUs. But not for years.

And yet extensions work on ChromeOS on machines with no more power or memory than my phone. In fact my phone beats my ChromeOS machine at many benchmarks, has the same amount of ram, and more storage.

Larger laptop form factor provides better sustained performance due to better cooling, as well as larger batteries.

Also, even though you can run Android apps and Linux containers these days, Chrome OS is still built around using the browser in a way Android is not. Your phone has to balance the needs of the browser against other demands and apps.

And you probably need to run a virus scanner and it eats batteries for breakfast.

It’s all trade offs....

It takes some doing to get a virus on a Chromebook.

Forgive my skepticism that that is the reason Google doesn't make it available.

Almost! But not quite. Extensions work fine on mobile Firefox and battery life and performance are improved when ads are blocked.

Anecdotal, but my 4 year old Android phone (Sony Z5 Compact) runs Firefox and uBlock Origin with no issues at all.

Probably not because both given reasons are plainly wrong.