Because a cylindrical shape is more stable and less prone to breakage or damage from external forces.
The tree trunk must support the weight of the entire crown and branches. To prevent breaking under harsh conditions like strong winds and heavy snow, it must be cylindrical.
Perhaps in the earliest times, some trees had non-cylindrical trunks, but they were eliminated through natural competition.
So now, all we see are cylindrical tree trunks.
The evolution of the spine into a circular shape is absolutely aligned with nature and adapts to the Heavenly Dao as a form of evolution.
The vertebrae, however, resemble a triangle, which is also a result of natural evolution.
The triangle is the most stable shape.
The vertebrae must support the entire upper body, so they need to be stable and robust.
