Because why use the most stable, busiest and most cost-effective launch platform to launch your sats? Amazon aren't sat on endless cash for nothing y'know?
Amazon aren't going away any time soon, doesn't mean you have to run your business like it's a government with masses of wastage and poorly-selected contract partners. You /could/ do other, interesting things, with that money and I suspect some would argue you could pay your warehouse staff more - but what do I know, it's a free market and whatnot..
I talked about this with OneWeb when the whole Russia thing blew up, people thought I was nuts for suggesting that they use SpaceX (and should have been all along) - like it's bad to make your business more streamlined on costs or use the best available launch platform, you're not in competition with SpaceX you're in competition with Starlink and Starlink is going to exist regardless of if you use SpaceX to launch - at least keep your costs down and successes high and give yourself a fighting chance.