Because in a fully remote environment it is easy to default to having transactional relationships and only when a job responsibility requires them. Just like I do with a customer service rep.
You don’t see your coworkers in the hall, overhear them talking to their kid, or talk while working. Certainly don’t by default interact with folks who work in parallel.
Not saying that in office means these things will certainly occur. But because by default these interactions don’t occur, the likelihood of them happening organically is quite low.
