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.

It keeps users in the app.

Probably has a lot to do with reducing phishing attempts too.