Because the experience of "installing" a web page by visiting the URL is still orders of magnitude more pleasant than installing an app. No need to clutter your homescreen with an app that you'll use twice and then forget about. Alternatively, no risk of not putting on your home screen and then forgetting about the 600 MB it's silently occupying on your disk until you're midway through your sister's wedding and trying to quickly free up space for more photos. No permissions to grant. No contributing to the endless stream of update notifications. No running in the background eating battery or ferrying your contacts to their servers or listening in on your microphone to use your voice for training their machine learning models.

Mobile websites are easier to install, fine to use for anything that doesn't require specific hardware permissions, and then when the user stops using them they stay gone. Not to mention they work on my Linux machine and my Windows machine and my iPad and my Galaxy. Forget apps.