the free ad space on your home screen. Sure it’s a small ad, but you see it all the time.
notifications. Even if only a small fraction of users allow them, it’s a lot of free advertising. And yes, you can put notifications on websites, but that’s not as reliable or as expected as native app notifications.
permissions. The more legitimate apps may provide some sort of additional functionality that their website can’t provide on its own. The shadier ones sell the data they get from the sensors all over your phone.
data storage. Technically web storage is a thing, but it’s definitely not something you want to hang your whole business on right now.
integrations. You can integrate, for example, Google Pay/Apple Pay on a website, but it’s more of a hassle. In an app, it’s practically drop-in. Same with the share functionality.
why not? If you already have a mobile site and can make an app from it reasonably easy, there’s no reason not to. You’ve become multi-channel with no extra work.
There are probably other reasons, but those are the ones that make sense to me, being in the industry.