Cost.
Front shocks are designed to take care of the bump(s) via VERTICAL articulation. Not LATERAL articulation. Fork seals/sliders and sleeves are designed to accommodate the VERTICAL function of the shock.
You ever see the residual rings of dirt and oil that will commonly appear on a front fork stanchion (the shiny part)?. Those dirt/oil rings are now gone since installing the fork brace. There is no lateral "flex" with the seals/dust covers and the fork stanchion. No oil will weep out since the seal is not being flexed laterally, thanks to the brace that holds the top of the shock tubes more firmly than before.