They never should have, and I'd say it's probably easier in some respects with smartphones because, while there's a multitude, they're all fixed, and you can generally target, say, 3-6, and cover a huge variety. And people generally can't change their font sizes.
Compare with dozens of monitor and window sizes on desktops. 13, 15, 19, 20+, with varying types of DPI and available fonts on systems. Not so much monitor sizes I mean, but... is the browser window full screen, or partial? People resizing windows would lose info, or not see it in the first place, and get lost. Back in the early days the "256-color palette" was considered a requirement for many projects.
While it's a hassle to develop for various size phones, it's still a more controlled and uniform set of sizes, imo; it just takes a lot more work.