Because cashiers are a different cost-centre, and thusly a different budget on the company's financial statements. The VP or senior director that controls the cashiers' services would end up looking bad if they had to retrench on that decision, and "looking bad" is death at that level.

A lot of what happens inside a company makes more sense when you realize it's a power struggle between a bunch of narcissists and their lackeys, and that VPs and CEOs aren't really as powerful as you'd think. Companies can be as inefficient and cut-your-nose-off-to-spite-your-face as any non-profit or public sector employer is, but we often don't see it because we've been trained to assume that "private sector == well-oiled machine" and "public sector == clusterfuck".

I was waiting to self checkout at a Walmart where there are 4 of around a dozen self-checkouts working. I asked about the ones that were not working and the employee told me that they can only open 4 units for every employee present. In order to have all 12 open they need 3 people there.