Take Wikipedia for example, even though it's changing very fast, I imagine a huge portion of it is static. If the latest version of each page was served via IPFS, somebody could setup a local mirror and people nearby would automatically use that.

Why would nodes in the BitTorrent network voluntarily act as a load balancer for your movies?

Same reason. Maybe you and I cannot explain the reason, but that does not mean that it doesn't happen. People don't always act in a 100% rational, for-profit manner, no matter what economics textbooks tell you.

(Edit: Of course the likelihood of your content being mirrored rises with the quality of your content.)

(a) People currently downloading and viewing the site. Like bittorrent, the more popular a site/page is, the more people will be seeding it. (b) The Internet Archive.