This ignores the first part of my response - if I, as a legitimate user, might get caught up in one of these trees, either by mistakenly approving a bot, or approving a user who approves a bot, and I risk losing my account if this happens, what is my incentive to approve anyone?
Additionally, let's assume I'm a really dumb bot creator, and I keep all of my bots in the same tree. I don't bother to maintain a few legitimate accounts, and I don't bother to have random users approve some of the bots. If my entire tree gets nuked, it's still only a few weeks until I'm back at full force.
With a very slightly smarter bot creator, you also won't have a nice tree:
As a new user looking for an approver, how do I know I'm not requesting (or otherwise getting) approved by a bot? To appear legitimate, they would be incentivized to approve legitimate users, in addition to bots.
A reasonably intelligent bot creator would have several accounts they directly control and use legitimately (this keeps their foot in the door), would mix reaching out to random users for approval with having bots approve bots, and would approve legitimate users in addition to bots. The tree ends up as much more of a tangled graph.
I've personally lived in places where the closest convenience store was 2.25 km, and the grocery store was nearly 18km, as well as places where a convenience store was literally a part of my building, and grocery stores were walkable distances.
The U.S. is enormous and varied. Take a look at truesizeof and compare the U.S. and Europe (don't forget to add Alaska and Hawaii - they won't be included in the contiguous states). Consider how different London is from rural Romania.