what you are missing is that he should not know how much of that money the recipient receives

imagine you are buying a new music album for $10

you type in $10 and hit send, and the merchant gives you your product

but what if the merchant *didn't* receive $10? What if $5 of it went to the merchant, $3 of it went to the original artist, and $2 of it went to the producer? That's none of the sender's business and the sender shouldn't know about it

He should just know "I sent $10 and I got my product"

He should not know how much the recipient receives

On monero, he does, and that is bad for recipient privacy because it exposes information about the recipient that the sender should not know

Lightning is better: the sender does not know how much the recipient receives, because lightning allows us to atomically split a payment among different people