This is not exactly true, or at least it's misleading. I recommend reading some Stuart Russel's articles on this or watching some of his talks. In summary, per

This is not exactly true, or at least it's misleading. I recommend reading some Stuart Russel's articles on this or watching some of his talks. In summary, performance is getting better on the high end, but at the same time, large parts of the world are coming online with terrible devices, so median device is not very good. P75 device is totally removed from what web developers use to develop on. On top of that, there's still a big concern with battery life, spotty connection and apps competing over phone's resources. E.g. on Android, if your website is too heavy, the system will kill stuff running in the background. If your users watch youtube or listen to a podcast and your website make the audio glitch or kill running spotify, that's not a great experience. Last point is that react-like sites use the phone's resources in a really bad way - phones get more cores (especially e-cores) and special processing units (ai, video, graphics) but JS runs single threaded on the UI thread, competing with everything else, together with expensive JS engine machinery. If you ever profile some SPA on some representative android device, it's just all yellow, bottlenecked on JS.