Why is input offset voltage so bad? (op amps)
why not just measure the current by letting Vos be the reference voltage in our voltmeter?


It's not really clear what you mean by this, but Vos is entirely internal to the op-amp, and it might be a combination of different errors in different stages of the amplifier. We only model it with a voltage source in series with the input. But there's no one physical place you could put a multimeter probe to measure it.


  How about other easy ways to calibrate-out the effect of the offset voltage?


Usually you can calibrate it out, for example by measuring the amplifier response to 2 or 3 different fixed input voltages.

One problem, though, is that an op-amp with high Vos is also likely to have a higher drift in Vos with temperature.


  Wouldn't those ways be much cheaper than buying a high-quality op amp?


Calibration requires additional test operations in manufacturing. These might require additional operator handling. That adds significantly to manufacturing cost. If you need to calibrate over temperature, it could add dollars (not pennies) to your manufacturing cost.

Then the calibration data needs to be stored in the device somehow, and retrieved to apply the calibration to each measurement. For some (many) products that's no extra cost, but for others it might mean adding an EEPROM and uC that weren't needed before.

If there's an error in the calibration process, or the stored data is corrupted, you get field returns, which are costly.