Because on lightning it is dangerous to go offline for a long time. On lightning, your funds are in a multisig address where you have one key and your channel partner has another.

Suppose you want to pay someone $5 in a channel where you have $100. To make that LN payment, you first make a contract with your channel partner. The contract says you will quickly let your channel partner have $5 as long as he shows you the preimage to the payment hash of a certain invoice. To enforce this contract, you use your entire channel balance as collateral -- all $100. So if you act slowly, your channel partner gets $100 from you instead of $5 as a penalty for breaking your promise that you would act quickly.

But LN payments can be delayed by anyone along your route, including your channel partner. So if you are using Mutiny, and your payment gets delayed, and you close the app, then -- when your payment finally goes through -- your channel partner will (try to) show you the preimage (but your phone app can't see it because it is closed), force close your channel when you don't respond (this triggers a kind of countdown during which you must let your channel partner have the $5 before the timer runs out), and -- if you still don't respond -- sweep your entire channel balance as a penalty for breaking your promise.

Your channel partner can do this even though LN is non-custodial because, when you clicked Send, you signed a transaction allowing your channel partner to do this. Once you sign a transaction, don't be surprised if someone uses it to take your money.

Because lightning.