While average individuals do have responsibility, theirs is many orders of magnitude smaller. Not only from their personal impact being comparatively minuscule, but because their objective possibilities to make substantial changes is negligible.

It's not "a matter of how you choose the percentage": the distribution is as it is, a "hockey stick" shape. The numbers just help to understand that.

Western people are far from being uniform in their wealth. While generally certainly substantially richer than other parts of the world, their societies nonetheless are wildly uneven in their wealth-distribution. You might want to look at actual data on the subject, your intuition is way off.

You're right on principle in that collective action is needed. But the reality of the matter is, collective action in western societies is captured by the rich. They control the media, the purse and the government effectively.