Because, like BTC, it is inconvenient for small transactions.  And just like ancient "money order" services for the last 4000 years, you could transfer "AUs" between Lightning wallets like BTC, with periodic squaring up via physical transfer (more trouble than a BTC transaction, so maybe less often).

Ancient services had only direct channels because the manual accounting was too onerous for indirect transfers, but a Lightning network automatically does the accounting for any path through the network of channels.

When there is no internet,  Lightning channels are down as well, but physical gold you have is still good, but BTC is useless.

BTC is up to 610G for self-custody.  The cost of running a full node (never mind mining) has been steadily growing, making BTC self-custody steadily more elitist.  Gold self-custody has been the same for thousands of years. 

 Ok, boomer.