I've never been to a grocery store where the self checkout doesn't weigh everything. That's why people keep getting the "unexpected item in bagging area" error that requires an employee to come over to check and clear the error each time.

Sounds like a stupid system.

What stores in what country are you referring to?

Pretty much every supermarket in the Netherlands.

Here is a video of it in action

The anti-theft equipment for a system like this that would prevent someone stealing by simply not scanning something is probably a lot more expensive than the usual self checkouts.

There is no anti theft system other than randomized bag checks where they check up to 10 items from your bag to see if you scanned them. Takes about 1 minute and with daily supermarket visits this happens maybe once a month or so. (I think there is some kind of reputation system linked to your store loyalty card).

Do you exit the store through a specific gate that scans stuff or what?

You scan your receipt af the exit gate (you can also scan a barcode from the store’s app or choose a tiny receipt that only contains the exit barcode). You have to go through one or these gates regardless of wether you go through self checkout or not.

If the store is busy I never try to self checkout since there are lines at all of them

There are almost never lines at self checkout. There are 16 self checkout stations vs only one regular cashier. Self checkout is super fast and even if they are all occupied one usually frees up in less than a minute.