Because it's meant to be a store of value. If I want to use it I want both privacy and fungibility and that's why I always end up spending Monero.
It is an option if you are comfortable using it. I use it once in a while. The good thing about this crypto world. Everyone is free to pay with the currency they want. I am a user of both LN and onchain. I still prefer to use Bitcoin and I try that this use is not at least never have anything to do with kyc. There are the tools within Bitcoin to have privacy. They are not a silver df bullet but there they are.