The same reason my Bitcoin cold storage hasn't been hacked.
Lightning is just a network of smart contracts connected to settled Bitcoin transactions.
Now, I can't say without 100% certainty that there isn't some unknown glitch or bug that would allow someone to spoof their channel moving funds in their favor. I don't have the development credentials for that. But given the sheer volume of testing being done if such an exploit was out there it likely would have been found already due to the size of the bounty.
Exchange hot wallets have massive honey pots, usually stored in one place, connected to a series of exploitable security mechanisms that exchanges have to use in order to withdraw from constantly.
Exchange security is proprietary and not shared with the public. Lightning on the other hand is a distributed open source tested network which gives it a major advantage. Security through transparency instead of obscurity.
In order for funds to move you have to sign transactions on your end just like a Bitcoin transaction. You're in control of your own funds and channel.
LN is still obviously highly experimental but it's starting to take off and if you're concerned about keeping your funds in a hot wallet then don't until we've seen a long period of heavy use with no major issues.
If I got something wrong someone please add/correct me.