The problem lies in cases where someone takes an image from Pixabay and says nothing and presents it as their own. When you let's say grab a photo of a bird and post it up with the title "Bird in park" you're giving the impression that you've taken the photo yourself and curators vote based on the quality of the photo. However, when they find out the bird is from Pixabay, they're feeling cheated. There is nothing else in your post except that bird pic and they just curated you based on someone else's work. So immediately they will either downvote or report. In those cases where the image is the sole point of value in your post, sourcing is highly advised (just say "I found this on Pixabay" and add something original such as why you're sharing it so the post makes sense).

However, if you've got a long post about your life or thoughts and you want to grab a Pixabay image as a header, maybe put the title text over it, that's a whole other matter. That's perfectly alright.

In your post, its clear that that's a stock photo. No one is going to assume that you've taken it.

However, people do want to see citations for images these days because of my first example. It's just the norm that built up after all the deception by those intending to scam where we became conditioned to look for sources.