Why don’t all wallets coinjoin by default? Is there resistance implementing this?
Zoom out to see our coinjoin of the week! This behemoth transaction contains 23 BTC - https://mempool.space/tx/927a4d5f3e17faae611f623eaf06206b966b30ba1bacbec49d7ab35afa50dbca
Bitcoin is divisible. WabiSabi coinjoins make Bitcoin divisible privately.

Discussion
It's just bloddy difficult...
We're trying to make it more accessible for devs with the NWabiSabi library and the Wasabi daemon.
There's a revenue incentive for the devs though, so hopefully we will see more adoption soon.
Zksnacks.com/coinjoin-api