Why do online retailers frequently NOT link directly to the shipping provider for tracking info?

Don't judge, but I buy a lot of stuff online. I try to be good and not buy literally everything from Amazon. But I have noticed that very often, online retailers wrap the shipment tracking system in their own garbage frontend. Even if the item is being shipped via USPS, for example, they don't just link to the USPS website in their notification e-mails. They have their own page that tries (and often fails) to show the same information.

Why do they do this? Is it to serve ads? Is it to track my data and sell it to data brokers? What could it be? Any ideas? I just wish they would link straight to the USPS, Fedex, etc tracking page so I don't have to manually do the two hop thing with the tracking number. You know what I'm talking about, right?