There are a bunch of reasons people/companies like web apps:
1 - Everyone already has a web browser, so there's no software to download (or the software is automatically downloaded, installed and run, if you want to look at it that way... either way, the experience is a lot easier and more seamless for the user)
2 - The website owner has control of the software, so they can update it and manage user access as they like, and it's easier to track users and usage that way
3 - There are a ton of web developers out there, so it's easier to find people to work on your app
4 - You ostensibly don't need to rewrite your app for every OS, but may need to modify it for every supported browser