It all comes down to risk. The longer they can sit on the money, the less risk. Stripe's entire business revolves around risk management, hence why it's 7 days, which is longer than others (their banking partners).
Is this the only reason bank transfers take so much longer in the US than UK? Why are UK banks happy to take the risk? (I'm genuinely curious and don't know much about why the US is slower.)
It's the risk of the credit card charge, not necessarily the actual transfer of money itself to the merchant. It takes likely takes Stripe a few days to get the money, and in order for them to operate the way they do (sign up and charge cards right away), they eat the risk of rogue merchants and other issues like chargebacks.