Why is dropping off passengers on the ramp by bus not more common?
The use of a jet bridge has several major advantages over loading passengers from the ramp via portable staircases or built-in "air stairs", among them:


Boarding of handicapped passengers
Not all passengers can use stairs: a jet bridge allows passengers with limited mobility, particularly those confined to wheelchairs, to board and disembark from the same level as the passenger deck.
Accommodating these passengers from the ramp would require some kind of lift mechanism to raise them to the passenger deck of the aircraft - this is typically accomplished elsewhere in the terminal when a jet bridge is used, e.g. via passenger elevators in the ticketing area, or in the type of design shown in your question, via an elevator in each "mini-terminal".
Passenger shelter
Boarding from the ramp may work well on a beautiful spring day, but in less favorable conditions (rain, snow, temperature extremes) most passengers would not find the experience to be enjoyable. The jet bridge provides an enclosed and relatively climate-controlled route from the terminal to the aircraft.  
Passenger Safety
Airport ramps are inherently dangerous places: Aircraft are starting their engines, being pushed back for departure, etc. and ground support vehicles are moving at all times carrying luggage, cargo, catering supplies, etc. to and from the aircraft. In addition to the ongoing movement there is a great deal of noise from these operations.
Bringing passengers into this environment represents an inherent liability risk: Someone could be struck by a support vehicle, or injured by jet blast. A jet bridge is an enclosed area away from these dangers, providing an improvement in passenger safety.
Airport Security
A great deal of effort goes into ensuring that unauthorized persons do not have access to the airside facilities at an airport. Bringing passengers into this area represents a security risk, as even a ticketed passenger could be intent on causing disruption or damage once airside.
A passenger wandering off on the ramp would represent both a safety hazard (as the passenger could be injured) and a security hazard (as the passenger could perform any number of malicious acts before they are located).