Why we don't see adjustable "High" and "Low" Voltages for discrete logic gates
You can have adjustable logic level thresholds if you really want to.  Older Logic Analysers use very fast comparitors that allow the threshold voltage to be set for the particular logic family and circuit that you are working with.

You can still do that today if you really need to.

But for general-purpose logic design, why would you want adjustable logic levels?  The TTL standard has served well for the past many decades and is still applicable today, even with new chip families that weren't even dreamed of 30 years ago.

The actual logic levels inside most chips does NOT rely upon a specific voltage reference.  Instead, the circuit is carefully designed to have the appropriate logic level thresholds for the particular manufacturing process that is used to build the chips.