Well, these people were lucky to have you. Unfortunately, many people just want to not be disturbed, and it's much easier to ignore someone through Slack than in person.
That's a question of culture and organization. Onboarding a new hire is a job, and someone has to do it. Same for mentoring juniors. If you're relying on in-office for that, because the newbie needs to tap physical shoulders to get attention, then I'd argue the setup is not good to begin with.
I've spent years now in a fully remote team, and we had no problem getting people integrated and productive, and bonding everyone into a real team. Likewise, I was the newbie in in-office teams before and have felt like an outsider who needs to beg for help for months.
In my experience, remote or not is not the deciding factor for onboarding and team building.
