Boring answer: “If it ain’t broke, don’t fix it.”

Getting new software developed takes time and money, and requires more training. Not to mention the risk of new glitches that weren’t present in the old software.

Restaurants typically have fairly low profit margins, so there generally isn't a strong desire to spend limited free cash flow to upgrade/replace a point-of-sale system that's functioning satisfactorily.

Especially since it's not something capable of a significant ROI since the PoS system usually has little-to-no impact on either revenue or profit. A slow one could impact productivity - but this is rarely a bottleneck so in most cases there would be little gained by spending money to upgrade to a newer system.

I think it depends on the restaurant. I've seen plenty with old non-NFC POS terminals and bespoke Windows 3x/9x-like UIs like you've mentioned that continue to work day after day. And I've also seen more modern restaurants using nice wireless Square terminals with clean associate facing interfaces. Probably just comes down to cost/benefit/training/setup etc.

It is expensive, and software takes time. It just works, and complicated systems aren't really necessary. Kitchens are super busy and people do not have time to play with things. Chances are they are running off a ten-year-old PC with QuickBooks. Over-innovating will detract customers. Users aren't techies. Take your pick.