IAS is calculated from the difference between the outside air pressure as measured by a "static" port on the side of the aircraft and the pressure measured by the pitot tube pointed in (hopefully) the direction of travel. It could be measured as pressure, but it is proportional to true airspeed under standard temperatures and pressures.

The short answer is that bus left about a hundred years ago.

See about page 10-24:

https://www.faa.gov/regulations_policies/handbooks_manuals/a...