I suspect that's because of limited alternatives. I still use promo sites for new releases because they are better than nothing. Even if achieving a positive ROI is difficult, the increased visibility may pay off in the long run. And no really great option has yet emerged to replace the promo sites.
You're certainly right about having to price the books low, which also complicates the quest for ROI. But it's a decent tactic if one doesn't overdo it.
I'll mention BookFunnel briefly because it does offer another avenue. A lot of people only think of it in terms of getting newsletter signups, but it also has good sales promos. Though BF does cost something, a paid plan gives you access not only to newsletter signup promos but also to sales promos, so if you're already using it, the sales promos come at no extra cost. They aren't world beaters, but with no releases in particular, the clicks convert pretty well to sales. Even a book that isn't a new release tends to get at least a few more sales, certainly as many as I'll get with a single promo site in most cases. As with the promo sites, though, $0.99 books do best.