Because as Greg says (and as the dog-walkers I saw exemplify), people are being educated to believe they are some form of protection which they are not. It would be far better to educate people to do as the man in black was doing; if they want to add reflectives as well, that's cool. The problem is that (like ever-brighter lights on vehicles) these are portrayed as 'safety gear' that replaces sensible practice. People think that as long as they are wearing reflectives (or driving with all their brightest lights shining), they are safe.
I do see my own visibility on a bicycle as a safety issue. When I choose to cycle in clothing that makes me more visible, I do so for my own safety. The gear that I wear for safety's sake is 'safety gear'.
Similarly, I have lamps on my bicycle which I switch on when the quality of light is poor. It's a safety thing. I take a road position that I believe improves my chances of being seen and avoiding unintended contact. I don't believe that by taking that road position I make all other safety issues superfluous.
I wear gloves and (very rarely) a helmet. That is protective clothing. It doesn't make me more visible, but it keeps skin and bone away from tarmac. It may be refered to also as 'safety gear', but its primary function is to form a barrier between me and other objects (also to make me look like a mushroom).
No-one has ever tried to 'educate' me to believe that the bright colours and the odd bit of reflective clothing I wear are protective clothing. Nor has anyone even implied the merest hint of a breath of a suggestion to me that 'safety gear' replaces safe practice. I'm not sure who in their right mind would ever make such a suggestion.
As to your last sentence, I dare say it's true. People think many things: People think the Royal Family are lizards and that it makes sense to import strawberries from South Africa. I dare say there are even people who think the Royal Family are South African Strawberries. But... no-one with more brain cells than a turnip really thinks that illumination and safety are synonymous. One may affect the other, but that's not the same thing.
I realise you're not anti-reflectives per se, but I really don't think people are so monumentally and unremittingly stupid that they'd believe (as Descartes did): "
Je brille donc je suis (sure)".
(From one of the great philosopher's lesser-known and less successful works).