I can't speak to Venmo/Cashapp, but Zelle does take backs. It's for specific areas of fraud such as the fraudster impersonating a government official/agency, or things like fake not for profits.
There are takebacks on Venmo. Someone recently paid me and then asked Venmo Support to reverse it. Venmo pulled the money out of my account without even checking with me. I'd have happily reversed the transaction if the sender had just asked me. If you're selling something to a stranger, nothing beats cash.