I think it's worth noting that fancier ways of showing loading usually take a lot more effort to implement so a loading spinner is a better-than-nothing default.

Skeleton UIs need to replicate and be kept in sync with your real UI implementation, progressively loaded images usually requires some server side pre-processing of the images and storing metadata, and calculating + receiving accurate server progress updates isn't trivial when the progress depends on unpredictable algorithms and multiple services.