No dumb question at all.

It's because every store has their own method for inventory, or creating orders, etc. So, it's simpler to just have the person at checkout enter the sale in their (probably legacy) system and then just type the final price in the PoS machine.

It's usually not worth the effort to link the two systems because there's no huge negative to doing this, and integrating the two system might not be trivial, and in some cases it would be impossible as some systems are simply manually done.

This has evolved to the point where card payments are simply done with a mobile machine. Now every store that accepts Bitcoin with lightning has something similar as well. Basically a mobile device with a screen where a QR code is shown. Some have NFC capabilities and other even have a thermal printer like the one you showed.

Here's one example in Germany:

Now, there are examples of where lightning has been integrated into their system. For example, McDonalds in El Salvador has the lightning payment integrated into their ordering screen, so it's even more seamless than the machine for cards.

Look how out of place the card reader looks here:

If you choose to pay with Bitcoin, the payment QR code appears in the same interface:

Bitcoin through lightning is clearly the future.