There's no one single answer to this. Some have been mentioned in other comments, but it's a combination of a few different things:

• Control: They have much more control over your experience as a native app than a web app.
• Ad revenue: It's significantly harder to block ads coming through the built in web views, and/or they can just build them in natively which is even harder.
• Integration: it's easier to do IAPs or subscriptions through native controls, which means less resistance, which means people are more likely to end up doing it.
• Data: it's easier to hoover up user data via native APIs than through the browser. There's way more accessible, especially if you can ask for a bunch of permissions and people don't notice/care. This makes any user tracking they do way more effective and any data they sell way more valuable.
• Notifications: Recently browsers have started adding support for this but it's not as effective. Push notifications are a huge boon to user engagement and this is a huge money maker. Having native notifications is a huge sell in this equation.
• Persistence: If you have your app on a user's phone, it ends up in the list of apps, meaning they pass by it very frequently. It's basically free advertising and living in their head without them even noticing. This is especially true on iOS where basically all of your apps are in your face all of the time.
• Performance: Native apps run way better and can look way better than web sites. If you just use web views this is mostly moot but still may make a small difference.

I'm sure I'm forgetting a few but you get the idea.

Websites are basically just inferior versions of native apps, and even if you use a hybrid/web view approach, you get many of the benefits and have the option to "upgrade" to a real native app later.

That being said, I fucking hate this shit. I don't agree that companies should do this, but it hands down does make financial sense. In a society entirely driven by capital and profit, it makes sense, but from a consumer perspective, it fucking sucks. I don't want to have to install the Facebook app to see some small businesses "web site" that's really just a Facebook page. I don't want to install reddits shitty native app to read more than 2 comments off a post about a solution to my problem.

It's legitimately consumer hostile, but company profits are more important than people in our society.