Maybe I'm unique in my usage of Instagram but I only follow people I know in real life or other photographers, amateur or otherwise, and they typically aren't posting sponsored content. I've only ever seen one person post sponsored content out of hundreds I follow, and it was pretty easy to unfollow them.

That's likely the difference. My wife follows people with similar interests, rather than only people she knows, so undoubtedly sees more of it.

Unfortunately it does happen. One of my coworker's mothers writes a food blog and he asked if she could follow me because she likes my photos. I said of course, and that she didn't need my permission. I followed her back out of courtesy, but then I started seeing occasional posts about "Certified Angus Steaks" and said nope. Good for her for getting money, I guess, but I don't want any of that.

I follow a lot of random people I don't know on Ig, but I actively unfollow accounts that start posting sponsored content.