I think: Instagram doesn’t allow links in image comments/captions, but does allow a link on your bio/profile page. So people end up sharing links by setting their profile link, and then commenting “link in bio.”
So the pitch here is, I think—make a full website to accompany your instagram posts, just as easily as you posted the photo itself.
It's by design. Instagram is another example of 80-20 (really 95:5) in terms of content consumers vs. creators/influencers. And the latter group are always selling some kind of product.
IG makes its money on advertising inside of the platform. As a result, they want to limit the ability of creators' to create their own ads by linking out to non-IG properties.
So the pitch here is, I think—make a full website to accompany your instagram posts, just as easily as you posted the photo itself.