That's a great question!

I would say a few things.

Number one, sender privacy with monero does have issues since you're hiding within only 16 people and someone with enough data can begin to eliminate some of these decoys to find you. This is being solved with an update to monero that increases the number of decoys from 15 to EVERY SINGLE TRANSACTION MONERO HAS EVER DONE. It's a huge deal.

Number two, yes lightning has pretty good privacy quarantees possibilites if you use it right. Unfortunately not enough of these things (bolt 12 invoices, proxy node in invoice creation for receivers for bolt 11, trampoline, blinded routes, MPP) are defaults making it harder for the user to KNOW they're getting the privacy they want unless they dig in and research and implement and use it in the right way. Not to mention SO MANY just resort to using a custodian for lightning due to it's absolute SHIT UX.

Number three, the use case for monero is several fold. It's relatively simple to use since most of the privacy is base layer privacy and privacy by default which is HUGE. Opt in privacy IS NOT privacy IMO. Not to mention it's accepted by a lot of vendors on the web since the monero sent to the receiver has excellent fungibilty. Something that onchain bitcoin just simply DOES NOT HAVE. All bitcoins DO NOT resemble all other bitcoins... which is really a shame. And not everyone accepts lightning. So for sure, if given the option by a vendor to pay in onchain bitcoin or monero, opt for bitcoin (or use a proxy like boltz.exchange/swap if you're against using monero for some reason). OH! and monero txs are cheap. That's something else going for it. Whereas large lightning txs start to cost you. And purchasing inbound liquidity will cost you. And FCs will cost you. Lightning costs tend to add up when you do it in a self-custodial way.