It's visible in the business practices: California has more McDonald's restaurants than any other state[1], and franchisees aren't running a charity. This law is the result of a compromise, as much as individual franchise owners may wish to portray otherwise[2]. If they're currently profitable and feel as though they can compromise at $20/hr, then I feel confident in claiming that they're realizing more than $20/hr in average employee labor value.
> But even if they did: why should the employer be forced to pay them more, when the job itself is unskilled and therefore has a surplus of folks demanding the position, even at their current rates of pay?
One good reason is to close an implicit subsidy: in states with high living costs, employees who require social services while continuing to work amount to an implicit labor subsidy on the government's part. I don't know about you, but I don't think the government should artificially depress wages like that.
[1]: https://www.scrapehero.com/location-reports/McDonalds-USA/
[2]: https://www.restaurantbusinessonline.com/financing/mcdonalds...