Why aren't voluntary response samples usually representative of the population

Voluntary responses simply means, someone approaches you and say "hey, let me give you my take on this or that topic", and they "volunteer" their response to you.

now, the people that will approach you, are just the people that will approach you, it could be all from one specific group, say all women, or mostly women, or mostly doctors, or mostly certain group, and therefore, they'll be giving their take on the topic, but for all we know they're just 15% of the whole population, it just so happen that they approach you.

and if they're 15% of the population, then their opinion matters only at the 15% consideration for the topic.