Now that I feel like I have enough experience to find a new job, I will be looking for a workplace with a strong emphasis on in-person work & collaboration.
I also observe that they are also the ones hanging out outside the office, so I would guess this is part of it too. Seniors tend to compartmentalize "work friends" vs friend-friends.
Edit: also a remote culture of hopping onto a one-on-one voice, vid, and/or screenshare helps too.
Ultimately I think people will work more in offices again for this reason and for the reason that even the company I work for, which has always been remote first has a large group of people who live in the same city, they meet in person more often and seem to call much more of the shots and receive promotions more than others.
We're already starting to feel the effects of the reduced talent pipeline especially as we get deeper into the Great Resignation.
For many early-career engineers living away from families in cities like SF, Seattle etc have better office spaces than personal living spaces - better buildings, better air conditioning, better TV, better restrooms, better kitchens, better stocked fridges etc. This factor does not apply as much to later-career employees who live in the suburbs.
In addition to the mentorship issue, think about promotions. Especially to more senior levels. The people in the office or with pre-existing relationships will be the ones getting promoted and will be the power brokers. If you are someone who doesn't think that will be true, then (IMO) you are confusing how you wish the world to be for how the world is.
I believe that most of these supposedly intractable problems can be solved with a Discord/Matrix/IRC whatever and just enforcing policies about communication. Have a bi-weekly required five minute voice chat and at least one hour of time per day where people need to have the chat open. If juniors are not asking questions or seniors are not replying within X hours or whatever follow-up sessions from the five minute intro discussions is appropriate, then they get warned, and if the lack of communication effort continues, fired. And you make that policy clear.
You can also look into various types of 'multiplayer' virtual presence. Several different browser based or 2d or 3d (even VR) options for this.
I feel like people just stop trying to communicate more than a weekly meeting or something a lot of times when they are remote. That in no way proves that different types of communication or mentorship or whatever require face-to-face. People just need to show up online to communicate.