How much time time does it really take? Let's compare it to a single check with with cash as the form of payment. Some diner at the table needs to go through the bill and and collect the money or else the bill gets passed around and then everyone antes up their portion, there always come confusion about what something on the check is or who had what. Then you need to collect all the cash and make change for everyone. Then the server needs to settle the check at the register.
Is that really any lengthier than just splitting up the check for individual diners? I am doubting it.
This is where the time becomes an issue. Running each card can take awhile, and when you add up 4-5 cards, you get a real impact on time that the server can't be taking care of anything else. In a job that doesn't have a lot of downtime, 5-6 minutes tied to a computer can be detrimental.
So 1 person pays then says "Everyone venmo me $X" and the party leaves.
You'll often find people at a table fight each other to pay for the entire bill.
Back in Slovenia bill splitting doesn't exist. To me it's a completely American thing. The best they can do in Slovenia is charge everyone individually for their items, but that's not really bill splitting.