Bugs are the responsibility of the process that brought them there. In most cases it is shared responsibility. Why doesn't your dentist mess up 1 out of 5 interventions? They have gone through years of strict training and there are giant liabilities in place. In development? A lot of self-taught developers, who can always jump to a next gig because of never-ending demand. This sets up the scene for a totally differen…
To me it seems that failures happens pretty often, generally due to incompetence, limitations in the science regarding the problem or insufficient resources. Seems very similar to bugs to me.
The question is how bad is failure, obviously if someone's health is at risk, you have a much bigger incentive to have processes on the safer side.
Maybe you feel "bugs" are more frequent in software because the software you're referring to has an incentive towards innovating rather than stability.
I'm sure if you studied software used in critical systems, the failure rate would be more in line with a procedure at a dentist. You can't justify the kind of cost that kind of development has for most apps.