My pet theory is that tipping became more common by almost accident. As the proliferation of various kiosk-based credit card processing machines took place (Square, etc.) where a little screen gets shared with a customer, the 'tip' step was either so easy to be turned on by the store/merchant OR it was even defaulted to on when the merchant applied for their credit card processor account and then the merchant just kinda left it that way; not enough people complained loudly enough or cared to do so, and so it stuck. Repeat tens of thousands of times across stores that opened or switched their CC processing over the years, even for places that didn't traditionally accept tips, and there you have it. Tip tip tip everywhere.