I have already answered this question on the Discord server.
In theory social login could be done in conjunction with posting authority.
The main issue is that now steemlogin would have to act as a proxy for all Steem requests.
In terms of bandwidth, five 9's availability, scaling etc, there are significant additional costs associated with this solution which I cannot cover.
From an engineering perspective it would require much more significant resources, which unfortunately are limited on my side.

The question is, why hasn't SteemConnect already added social login?
If tomorrow social login was added to a better alternative than the above solution, such as build-in social login within Steem nodes (never going to happen) or Steemconnect, I would be the first one to phase out Steemlogin.

The other point is that for many the risk of sharing their posting key is acceptable.
It is up to each individual to make this call.

What I know for a fact is that there are many Steem app developers that have been discouraged because new potential users were put off as soon as they were presented with a SteemConnect screen asking them to enter their private active key, especially on mobile devices.

The lack of massively popular Steem apps out there and the stagnation of Steem in general is proof that things are not right at the moment and some shaking up needs to happen.

We all want for Steem to become a widely adopted platform for many types of apps, and SteemLogin is probably far from being the best possible solution to the sign in problem.
But I believe that it is a step in the right direction and, if nothing else, it will trigger other solutions which will improve ease of access for the general public.

Thank you for your feedback.