How hot is 8?

Why don't stoves have a temperature display? I asked this about toasters a while back to my dad on why they have numbers ranging from 1-10 instead of a temperature gage. He responded “it is because the filament wears out over time so what used to be 6 is now 8”. It makes sense, however on a stove it does not. The part of the stove that heats up shouldn't wear out like a toaster filament. Even if it does wear out over time that still doesn't explain why they don't put a thermometer of some sort in the unit that heats up and will read the temperature back. So instead of deciding whether 6 is hot enough to boil water, one can actually punch in the number 100. Showers suffer from the same problem. I spend the first 2 minutes of any shower just trying to figure out the right temperature. Why not put in some kind of system so when I turn on the shower I turn it to 55 degrees, or whatever temperature I want. Instead of having to figure out how much of the cold handle should be on and how much of the hot handle should be on.