Why does sanitizing a document turn text pages into images?

I just used the Sanitize Document Tool for the first time on an example file here and it didn't turn text into images here.

Thus, the behavior you observed is not universal but instead is likely to depend on a property of your document.

What this property of your document is, is hard to tell.

If I had to guess, I'd go for some form of transparency effect. As mentioned, though, that's a guess.