Because you bought the non-'sport' model and this is the dashboard you got with it:
The tach was optional; you got an idiot light farm, a fuel gauge, speedometer and an odometer. You also couldn't hook up most service tachs to the then-new distributorless ignition system and OBD-II wasn't a thing yet, so you either had to hook up a giant diagnostic computer to the vehicle or have some other provision for tach readings for diagnostic purposes.
Thomas' Cadillac has a similar data display set.
Going to an all idiot-light-farm setup like this was common back then for the Detroit Three. It was initially well received by the general buying public, but then became much ridiculed.
If you shelled out quite a lot of additional money, you'd get this dashboard in your Olds instead: