Why is my available balance lower than my wallet balance?

Because bank payouts you have already started are holding part of it.

When you send a payout that your wallet cannot yet cover, it is created and marked Started rather than failing. It waits, and it sends on its own once the funds are there. Until then its amount and its fee are reserved, so that money is no longer free to spend on anything else.

Stripe does not know about those payouts, so the raw wallet figure looks higher than what you can actually commit. Available to send is the wallet minus everything waiting, which is the number every payout is checked against.

You can see which payouts are holding the difference in your payouts list: they are the rows marked Started. Only bank payouts appear in the reserved figure, because PayPal and Venmo are paid from your own PayPal account rather than from this wallet.

A waiting payout still needs the customer's bank details before it can go, and one left in Started for 30 days expires and has to be created again.