If it's as easy as that, I'd expect merchants to adopt them everywhere. That being said, I do know that the rate of adoption is increasing, so that's good news, but there are challenges to set up and get accustomed to that.
It's that easy bros, the reason this method is not used by mainstream merchants is because they don't know about it, talking about LN is too far, I don't think those merchants know what BTC actually is, still thinking the same thing as stocks.
Even running their own payment gateway or opening an LN channel can be a problem for new users. I think it's not a bad idea to make them even easier to get into, as long as it doesn't sacrifice important aspects like security, privacy, etc.
To make BTC transactions, we can now do it without Bitcoin Core & all its synchronization stuff. Also applies to LN transactions, to send & receive, we also don't need to open our own LN channel. Don't waste what developers have built on platforms like: BlueWallet, Muun Wallet & Coinos.
