As for the bike, I do a lot of road cycling, which don't have ANY mirrors, so it's just habit now. You don't check over your shoulder when your're riding and you can easily get hit by a car.
You are correct if stating they don't usually come with mirrors as standard equipment
(some do). As an avid cyclist for many decades clocking well over 100,000 miles (yes I meant one hundred thousand) I can attest the circumstances are quite different. I have crossed state lines on journeys of 1,200 miles and more, alone, and never had an incident involving another vehicle.
1) On a bicycle you are not riding in traffic on the highway.
2) A smart rider will be in the traffic lane at intersections but otherwise will be to the extreme right or in a bike lane if provided. This means cars will only be to the left side, never on the right.
3) The speeds are much slower so distance traveled while looking back is much less.
4) A truly smart rider realizes mirrors are available for bicycles, purchases them and learns to use them as intended.
Note: Figured I better add this as some posters like to nit pick. Yes I realize going down hill we, I, have attained speeds of 70mph and more, but these are special circumstances and at these times I'm usually only concerned with the vehicles I'm passing
while breaking the speed limit. And yes, even on a level surface I've axceeded 40mph but only for short bursts so this doesn't apply as a general rule. Please refer to Item #4 if you still don't get the point.