The viewer login page is displayed before the user is logged into the viewer. So it can't be used to display user-specific information such as your own online friends, but rather the names of everyone who is currently online. Letting people know you are inworld when those people aren't your friends is both a privacy risk and enables people to more easily harass you (if that is their intent).
We could add online friends information to the website itself when a user is logged in (i.e. not to the viewer login page) and I agree that that would be nice to have. Unfortunately, the list of similarly nice-to-have features is long and they, bug fixes, infrastructure improvements, operational work, and a long list of other tasks all compete for our limited R&D resources. So, while I agree that seeing your online friends on our website is desirable, it is just one of many other tasks that we have to work on, and as such must wait until working on it justifies delaying work on all the other things on our todo list.
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