Because it’s not sustainable and it’s not a real LN wallet. If you haven’t experienced high fees from it yet, you haven’t really been using LN a lot yet, especially larger payments.
Muun will make you wait for an on-chain confirmation when sending larger payments because they don’t trust you with a zero conf for larger payments.
Muun also doesn’t fully support LNURL yet.
It’s also a terrible on-chain wallet too, there’s no coin control and you’ll end up with a mess of UTXOs with the submarine swaps.
Try withdraw all your funds from Muun on-chain and you’ll see all the inputs get merged, and it could be quite expensive to do so.