Not sure about Yoshinoya, but I've heard that Sukiya is considered a "black" company. Means they run over people, understaff and, in general, it's not a place you want to work in unless you're desperate. That was few years ago, though, so not sure how is it, now.

You probably went during off time? It's usually more than 1 during busy time. (And it's Sukiya, not Sukiyaki)

And it's also probably not for wanting just 1 staff -- it's probably because no one want to work at those place.

I went once at about 3-4PM and also came away feeling like it was a bit understaffed. One worker and about 8 people in there eating. I didn't have to wait a crazy amount of time so maybe it's fine though, gyudon doesn't take a long time to prepare if some ingredients are prepped

By low-staffed, do you mean that you had to wait long time to get your order? These places are highly automated so they don't need as many staff, sure there's few people working there but usually more than enough for serving the orders that come through in just few minutes.

Yes it's very optimized so single person can handle operation, but I prefer to have at least two for backup and safety.