From looking out the window at US airports they just do not have the buses, or any bus drivers, or even the steps, available for use. And the investment in them (steps cheap, buses not) is probably felt not to be justified for the few occasions they might be used.

Most airports in Europe have handling agents who provide these services and many others to a range of airlines. In fact you pay a lesser handling charge if you do not use the jetway (hence why Ryanair never does). In the US airlines tend to do ramp services themselves, even at smaller stations (always seems very inefficient to Europeans but I believe there are union restrictions), and they would each expect to provide their own steps and buses. And as pointed out above, there is a cultural disbelief among US passengers about using steps. You should hear the moans when they are used at London Heathrow to disembark a transatlantic flight at a remote stand.

Maybe also, looking at some other FT threads, the statistical-average US passenger is nowadays too fat to be able to negotiate the steps