The reason is that above about 3.45V per cell, there is very little additional storage to be had. If I recall, it is less than 1% extra. Above 3.55V per cell, it is even less. A proper LiFePO4 cell can be pushed safely to 3.8V, but to what end, an extra 0.1% capacity? Why bother?
And as we all know, longevity is improved if you stick lower down.
That’s why 3.55V per cell is more than ample. You want it just high enough that balancers can do their job, and not much more.