It's less about DPI than about raw pixel count. At 1080p, you simply lose too much usable screen space to menus, toolbars, task bars, and all the dumb shit that websites set "position: fixed" on. 2560x1440 (or better yet, 2560x1600) with appropriate magnification gives you a lot more flexibility in how you use screen space, instead of more or less forcing you to work with only one maximized/fullscreen window at a time.

I use a 13" MBP with no UI magnification, but most web pages scaled to around 200%. That keeps the UI nice and compact and leaves a lot of screen space for actual content (about 2170x1523 with my current browser layout). I do occasionally set it to a scaled resolution when someone else has to watch/use the machine as well.