On average, less price sensitive people will tip more, but there are no guarantees. Somebody who’s wealthy may still tip 20% over somebody who is less well off tipping 25% because they’re feeling charitable.

Or, more commonly, some people don’t tip at all, even if they can afford it. Anybody who’s worked in food service has experienced this.

You wont find anything close to a linear relationship between wealth and tipping percentage. Most will tip 20% ish regardless, because that’s what’s generally expected

it's not explicit because no one knows what the total cost is that's being shifted to the consumer and how much of that is recouped by tipping.