[counters]

The measurements aloft constrain the entire system - things like vertical profiles of moisture and temperature, as well as the kinematic structure of the atmosphere (e.g. wind profiles) grossly constrain the evolution of the system. Put another way - the _information density_ of these observations is very high and they tend to constrain non-local features of the flow / structure of the atmosphere.

Observations for the surface don't have this effect for two reasons: (1) they can be dominated by local influences (like local topography) that poorly constrain the background atmospheric state, and (2) the majority of numerical weather models do not directly model the planetary boundary layer (the layer of the atmosphere closest to the ground), and instead parameterize processes that occur here. What this means, practically, is that the information content of surface observations is low (1), and even when it isn't, there isn't a mechanism to effectively propagate this information outside of a single grid cell or even column in the actual forecast model (2).

That's why observations are typically used to bias-correct forecast models - it's a form of localization or downscaling.