Why are bastions an individual character mechanic, rather than an adventuring party mechanic?

Love the bastion features and flavours, I've tried to home-brew a similar system for my game, where they slowly add features to their keep over time, and during a time of extended rest they can use their bastion to gain benefits (study, train, influence, craft etc).

One thing I'm curious about is why the rules are designed so that each character gains, maintains, and develops their own bastion seperate from their party? Can any one explain why it has been designed as an individual character mechanic, rather than a team mechanic? As a DM it seems unnecessarily complicated to track and engage with multiple bastions over a campaign, compared to the adventuring party either finding and claiming a structure, or being awarded one as part of services to a city/region. The rules say the players can combine their bastions, but why isn't this the default?
For a system that requires players to work with each other as a team, it baffles me that DnD has so few mechanics that reward teamwork. I was really hoping this bastion system would the start of introducing more team based mechanics (team feats, team items, team training during downtime, rewards for teams working together and strengthening their bonds).

Anyway, that is my only gripe, everything else looks really fun - I love the flavour of the books and tavern drinks - I would just change upgrading and points system to reflect team progression rather than individual player progression.