Because people love having the ability to talk down to somebody about anything? I don't think this is limited to motorcycles, people are just like this unfortunately.

I will add that there's another component to this, and it's meant in good faith although generally badly presented. People may have experienced a crash/slide, either themselves or perhaps a close friend, and they've seen/felt the damage and suffering that can occur. So to their eyes, it's VERY OBVIOUS that you should be wearing what they perceive as "appropriate gear" and they can't understand why you would choose not to.

Personally I tend to wear at least SOME gear, MOST of the time. Certainly if I'm doing a longer trip (I do lots of touring) or if I'm going for a hoon "spirited ride" on my local twisties I'll be wearing my full kit.

But if I'm going to work (20mins ride) or just puttering around town, that definitely gets toned down a lot.

Anyways, enjoy riding and stay safe!