Why would anyone use DOT fluid and vacuum-based brakes? Mineral oil is much less toxic than DOT, does not absorb water, has higher boiling point.

Bleeding Shimano brakes is almost as easy as unscrewing the caliper bolt and pouring oil into the lever system. On the other hand, for Avid-s one needs to observe strict vacuum.

Why would so many companies bother with an American-specific oil and vacuum-reliant system?

Mineral Oil
Shimano
Magura
Tektro
Gatorbrake  

DOT
Hayes
Avid
Formula
Hope
Advantages of DOT over Mineral Oil:


DOT has a standard - product that meets the minimum standards is readily available at low cost
Compatibility with cheap materials used for seals
Hygroscopic, so water will not collect in a single part of the system


Disadvantages of DOT over Mineral Oil: 


Corrosive despite inhibiting additives, damages paint, skin, metals etc.
Hygroscopic, so water is absorbed and the boiling point drops over time. Limited shelf life and operating life.