Payment processors like Stripe may fix the rate at something profitable for ease...

Payment processors like Stripe may fix the rate at something profitable for ease of explanation, but the underlying interchange fees are not the same.

They even vary by credit card type (rewards cards have a higher interchange rate than a non-rewards card), and by type of merchant. See the MasterCard rates at https://www.mastercard.us/content/dam/mccom/en-us/documents/..., for example.