Because that is an extra feature that is usually only put onto trucks when an O/O is building a brand new truck. Even then the feature is probably passed on majority because it cost weigh to much (heh, see what I did there?), and a cat scale weigh is only ~~15~~ 17 dollars. The closest thing we have to an "on board scale" as a standard feature is the air pressure gauge, which once you weigh enough times you can figure out the approx weight based off what the air gauge says. Like on my truck, so long as the drives are under 63 psi, then I'm under 34k on them. Once had a truck that I drove for so long that I could tell you how much weight was on the drives whether the gauge read 54, 60, or 66 psi. All I remember of that truck now though is that 64/65 psi was the limit before being over weight.