Because in the Lightning Network, payments require multiple interactions to succeed. Additionally, the transaction fees for self-hosted Lightning Network transactions are very expensive.

I don't understand. What do you mean by 'multiple interactions to succeed'? Transaction fees on Lightning are in the order of one hundredth of one cent.

There are no lightning stamps. Ecash is bearer asset, lightning does not have anything like that, but it's convertible, it is actually created by paying a lightning invoice. Doing several roundtrips to pay 1 sat (which would be requirement for lightning) is not feasible.