Just to put a slightly different perspective on it - the reason that splitting equally works is because it averages out over time - some you win, some you lose
It really doesn't a lot of the time.
Light eaters don't suddenly develop an appetite for 3 courses.
Vegetarians/vegans don't (usually) start eating meat/fish.
Tee totallers don't (usually) start drinking alcohol.
People on tight budgets may often not be able to spend more for many years.
And that last point is the biggest shitter in all this.
A person on a tight budget might be able to come if they have one of the cheaper mains and one drink. Their bill, including a tip, could be £20. They've limited what they've had, to stay on budget, and might have no more money available to add to the pot. If they spend more, they have to cut down elsewhere in order to be able to afford it.
Yet someone who's possibly had a starter (£5), a more expensive main (£20), an extra side (£5), dessert (£6), a bottle of wine (£20) and a coffee (£4) thinks it's reasonable to say, you know what, instead of you paying £20 and me paying £70, shall we just split it and pay £45 each?
Because I think you should not be able to buy a full grocery shop this week so you can treat me to this big meal I've had and I CBA to do a bit of simple adding up.