Before registers easily made separate checks including each tax amount it was easier to split evenly. Especially when paying with cash but even if some throw in their cards.
Often with splitting the payment without separate checks you’d get people that didn’t realize the cost of their soft drink or didn’t figure their tax or think of a tip.
Then either someone else would have to overcompensate by putting in more than their share or the tip would be tiny as everyone chips in too little.
Let’s say four people eat together and the total with tax come out to an even $40.
You figure $8 for a tip.
One person had an $8 meal and a water.
Another had a $7 meal and a coke.
The other two had similar variations.
Maybe the individual totals would be $8.50, $9.50, $10.50 and $11.50.
Their respective tips would be $1.70, $1.90, $2.10 and $2.30.
They would chip in $10.20, $11.40, $12.60 and $13.80.
You’d spend a while passing that bill around as everyone realizes their beverage cost and then figures their tax and then tip on top of that.
Or take take two seconds and decide to have everyone put in $12