The main reason is that it makes my life a lot easier, they act as a reseller so I effectively sell my product to them, and they sell it on to customers. At the end of each month I just need to enter a single payment into my accounting platform, and they handle all currency conversions (I sell in USD, my accounting is in GBP) and sales taxes. They of course charge more, but for the amounts I'm talking about the cost outweighs the time I was spending dealing with Stripe payments.
They also support PayPal which I've had a lot of people ask for, so that's a win too.