Is your question rhetorical? In case it's not:

1) Web applications make it easy to require monthly subscriptions, which many people don't like but is great for companies

2) Web applications can be more easily monetized through ads

3) Web applications can be updated for everyone more easily than a bunch of native installs, potentially reducing support costs

4) Web apps are a solution (not the only solution, but probably the easiest solution) to cross-platform development