Tire Sidewall is Where a Lot of Your Traction Lives
Foam inserts are good at absorbing energy, but they have to take up a lot of space to do it. They press against the sidewalls and change how the tire engulfs terrain. That affects traction, especially when you’re on the edge of grip. You may get more sidewall support, but sometimes that sidewall support can compromise other aspects of your ride that, to some, are more important.