That is, they do not posses the necessary understanding to comply with Regulation 16 of the Electricity at Work Regulations 1989, which states, reasonably enough, that,
I didn't need to look that up - I quote it verbatim several times each week.
I only use electricians as exemplars, because their competence is an area in which I have a pre-eminent expertise - I'm fucking brilliant, me - but I have no doubt whatsoever that the principle of majority incompetence extends far and wide and into every field of human endeavour. You are almost certainly less than perfectly competent in what you do and part of that shortfall is not knowing what you don't know, or not caring about how what you do affects others. (Most people 'cards in' fall into the latter category, seeing their job as a means to a pay packet and not as meaningful in its own right.)
But, at least electricians and many other trades and professions have regulations to fall short of. And training to attempt to meet those standards. Standards that are sometimes hard to meet. Standards that are often changed, with little notice and for no apparent reason. Standards that are sometimes quite difficult to understand in the first place. And most people - ignorant or otherwise - seek to put in a fair day's work for a fair day's pay.