I use it exclusively for announcements from certain brands with e.g. seasonal rotations or sales (small shops, especially, are often way more consistent about updating one or more social media accounts, often Insta, than their website, if they even have a website) and it's such a pain in the ass for that reason. I don't trust ads or their "algorithm" to promote quality (I reckon they're more likely to promote rip-offs and fly-by-night operations) so I super don't care about anything else they want to show me, even if it's directly related to the kinds of brands I'm following. I deliberately do not do new-stuff discovery in the app, because they have incentives to screw me.

The only thing I want out of it is to see the posts made by the accounts I'm following, since the last time I checked. That's 100% of the functionality I care about, and the app goes out of its way to not deliver it.

And the shops are on FB/Insta/WhatsApp only because that's where users are. Classic entrenchment of network effects is a two-sided matketplace.