I realized that we provide an option called "Flatten PDF" [1] which does exactly what you described: flatten everything including text into a big image per page. It makes all text unselectable and also produces a very large file.
This feature was introduced for printing consistency as PDF viewers and print drivers may render things differently. But I don't think it's selected by default, so users have to consciously tick it to make it happen.
But it seems that our support team sometimes presents this feature as a way to solve user issues, so maybe that's the reason.
"Flatten" comes from the world of desktop image editors. You "flatten" one layer down to the next to make one layer. Sometimes the people who work on these kinds of things forget their users might have never used software with metaphors like this.
This feature was introduced for printing consistency as PDF viewers and print drivers may render things differently. But I don't think it's selected by default, so users have to consciously tick it to make it happen.
But it seems that our support team sometimes presents this feature as a way to solve user issues, so maybe that's the reason.
[1] https://www.canva.com/help/download-flattened-pdf/