US states differ on minimum wage for tipped employees. For instance in Idaho, minimum wage is $7.25/hr, but tip-elegible employees only get $3.35/hr. In that case it would be very unfair not to tip.
I might be off on this, but I believe the California minimum wage is $14 across the board, going up to $15 soon…..and yet tips are generally still expected. Standard minimum seems to have gone up from 10% when I was a kid, to at least 20-25%. Now that paying with card machines are the standard you often can’t complete a transaction without choosing a tip amount on a screen, or hitting several buttons to opt out of tipping, even for a cup of coffee or takeaway food.
There is a large percentage of Americans in love with tipping and will defend it no matter what. I think it probably comes down to personality type.