Why do some volume controls start at -80db or..?

Reality is a good reason.
Since mmost environments have about 40 to 50 dB of background noise, having a realistic dB setting makes sense.

((Zillions are for fantasy games.))

Then the 'minus' is within the system. The ATTENUATION of the incoming signal is the 'minus' and the zero is the point where incoming and outgoing are (realatively) identical. Then plus dB is the increase over the level of the incoming signal.

Hope this helps to make sense of why it is done.

My Audia starts at -90. If you like things that start at zero let me recommend the Kelvin temperature scale; starts at absolute zero. Water boils at 373 K etc.

What Elizabeth said. On volume controls that use plus/minus, 0 is zero gain. Which is to say, at zero grain, the input signal from the source is effectively untouched by the volume pot. Now, for your average digital source, zero gain is real hot and damn loud. Thus, the majority of the usable volume range will be in the "minus" realm, in which the volume control is operating passively to attenuate (or apply negative gain) to the input signal. When you get into the positive numbers, the volume control is using a mini amplification stage to "add gain" to the input level and boost it above zero gain. (A "passive" volume control will only go negative, and have no amplification stage or ability to add gain). So, volume controls that display +/- are actually providing useful info that ones that read 0 to X do not -- ie, where zero gain lives and the volume control is doing the least/nothing to alter the signal. FWIW, I prefer the 0 to X range myself, but the other is perfectly legit and useful.