If you came in late at a restaurant or hospital, they'd be short staffed and screwed.
I worked at Target in the fast food restaurant some years ago. If I was late to that, there was simply no one to take orders. It directly impacts the income of the business.
Late once -> fired definitely needs to be fixed, though.
They're saving money by treating humans as perfectly interchangeable cogs.
This is why you have backup staff and don't drive people to the point of maximal efficiency. Because it's inhuman.
The industrial revolution brought GDP growth, but to a very small number of people. The rest (practically everybody) were plunged into abject misery. This is where anarchism, marxism, communism, socialism, fascism, and so on came from. I mean, one of Henry Ford's famous innovations was paying his workers enough to afford the things they were building. It was a capitalist nightmare.
WWI and the Great Depression unbelievably made things worse, and they led to serious political upheaval throughout the western world. It was only when WWII wiped out the holdings of many of the wealthy that quality of life started to improve at all, and in fact that lead to one of the longest periods of growth and equality in human history. But it really had nothing to do with economies of scale. It had to do with the fact that incomes finally rivaled capital gains from wealth.
The essential difference between that period of expansion and the initial industrial revolution weren't economies of scale. It was that we had political systems that distributed the profits to more than just the business owners. If you don't have those kinds of systems (free markets, progressive taxation, financial regulation, open credit), then it doesn't really matter how efficient your production is, because the benefits are only going to .001% of people. If those people aren't sharing (not exactly something rich people are known for), then quality of life for everyone else won't go up at all. And hey look, that's what happened during the industrial revolution.
Source: basically just read Piketty.
The individual player may not have a choice - that's why we attempt to influence the system such that these situations don't arise.
In FY2018, Walmart "generated $28.3 billion in operating cash flow and returned $14.4 billion to shareholders through dividends and share repurchases" [1]. The best study I can find [2] says a hike from $10/hr. to $15/hr. would cost around $5 billion a year.
TL;DR: Walmart could pay workers $30/hr. The money comes from there.
[1]: https://news.walmart.com/2018/02/20/walmart-us-q4-comps1-gre...
[2]: https://www.reuters.com/article/us-walmart-wages-idUSKCN0YW2...
Of course there are extenuating circumstances, and managers should be more relaxed with people who make an effort (e.g. texting ahead that they are running x minutes late). But the vast majority of cases are not like this. Its people making stupid choices and suffering the consequences, I don't have much sympathy for that.
The lengths you have to go to to avoid being late essentially boil down to "always leave 30 minutes before you have to". That's a huge, completely unpaid time sink. If I got "paid" an extra 30 minutes a day, I'd make $10k more a year. And sure hourly workers don't (usually) make as much as I do so their increase would be something like $3k, but that increase probably matters a lot more to them than it would to me.