The app https://play.google.com/store/apps/details?id=com.kii.safe is free, so no. Some users might not want google to know they're using a particular app (this one a is a security one, so probably a higher proportion of users would care). There are over 10 million users from the play store, so 100,000 is not that high for sideloaders.

Plus, google might block downloads in some countries.

There are a number of sites that offer apk downloads for free apps: see http://apkpure.com/, https://apps.evozi.com/apk-downloader/, or http://apk-dl.com/. See also http://www.makeuseof.com/tag/download-apk-google-play-bypass... for more reasons people might not want to use Google Play.