There is speculation about Instagram Ads affecting their algorithm placement negatively, which is why they want to avoid it.

When you use Instagram Ads regularly, Instagram demotes your posts to encourage you to keep spending. We've tracked this over the span of 6 months. During this time their organic impressions dropped in half

Once we halted their Instagram Ads, the account recovered back the 50% drop in impressions, but took about 8 weeks to stabilize.

Another friend of mine in the ad space says the same things happen with YouTube when using their advertising. So people are turning to Facebook.

Example: Uploading a video to Facebook, with a link to their YT video, boosting that video on the Facebook newsfeed = traffic on their YT video that does not demote their content in the YT algorithm.