The algorithm doesn't work for you
Instagram's Feed, Explore, and suggested-post surfaces rank and personalize what each person sees. That can be useful for discovery, but it is not a fixed archive of your work: distribution depends on signals and systems the platform controls, and not every follower will see every post. Strong work can still be hard to find once it is buried in a profile or no longer being recommended.
What a portfolio needs to do that a feed can't
A serious inquiry — a gallery, a client, a grant committee, a press contact — needs to see your strongest work in the order you choose, with the context you choose, without wading through unrelated posts, ads, or a feed organized by the platform. A portfolio is deliberately curated for evaluation; a social feed is optimized for ongoing sharing and discovery. Those are different jobs.
A site on a domain you control also reduces dependence on any one account. Instagram accounts can be disabled, including in cases where the owner asks Meta to review the decision. A website can have outages too, but control of the domain and backups gives you options to move hosts or rebuild elsewhere.
Instagram still has a job — just not this one
None of this means abandoning social media. It's genuinely good at discovery, community, and staying visible day-to-day. The fix isn't choosing one over the other — it's making sure the platform is a doorway into something you own, instead of being the entire house.