Sounds like they could be using your cash as funding. If they hold a bunch of cash from people it is effectively theirs for that period of time. This could prevent them from needing to raise a funding round if done enough. Some businesses are literally customer funded like these. If you want to read more search for "negative cash operating cycle".
We don’t touch reserved funds (and don’t earn interest off them either). The funds are saved for individual Stripe accounts and are only used to cover refunds and disputes from their customers.
Why not? You could use it to lower the cost of your services. (Someone is obviously earning interest on it. Better you than your bank.)