Because an electron JRE will run into the same versioning issues that the JRE has.
Not to mention the additional headache of having to install dependencies which can be an annoyance on some platforms.
At this point in 2016 the number of people that are negatively affected by an electron app's size on disk is significantly less than would be negatively affected by having to muck with installing "Electron Runtime version 52" for app A but need to lock it to "Electron Runtime version 50" for app B.
Is there really anything in your average Electron app that breaks when Electron is upgraded? I assumed that Electron apps would inherit implicit forward-compatibility from web-apps, allowing "Electron" to just always mean "the latest Electron" in the same way that "Chrome" means "the latest Chrome."
Also, I'm not so much concerned with size-on-disk as I am with
1. redundant versioning leading to redundant network bandwidth costs (e.g. I don't want to find out that I have 20 different apps that want to download a 100MB update—99MB of which is the same Electron binary—every time Electron itself updates.)
2. memory usage (30 versions of the Electron runtime means 30 copies of Electron taking physical memory. 30 apps using the same Electron runtime means 30 copies of Electron in virtual memory sharing the same physical pages.)
Sadly that's not the case. Many electron apps are written with a single version in mind and that version needs to be treated like any other dependency. Knowing you are targeting a single platform lets devs aggressively optimized that platform.
Also, some electron apps will modify the "browser" part.
As for download sizes, aside from the first download all updates can use Delta update packages and I believe this is built in.
Re memory usage, with sandboxing the way it is today I'm not convinced you would save all that much memory unless you were running over 10 different electron apps, and even then compared to the total memory usage I feel it would be insignificant enough to not warrant the "JRE architecture". I don't really have anything to back this up, its just a hunch.
Not to mention the additional headache of having to install dependencies which can be an annoyance on some platforms.
At this point in 2016 the number of people that are negatively affected by an electron app's size on disk is significantly less than would be negatively affected by having to muck with installing "Electron Runtime version 52" for app A but need to lock it to "Electron Runtime version 50" for app B.