> I wonder why there isn't a mobile OS that simply lets you build apps with JavaScript, HTML, and CSS and gives you reliable storage without all this effort.

There is! You just have to time travel all the way back to 2009 when webOS was launched by Palm. Time travel is the easy part, you then also need to somehow prevent Palms demise and webOS fading into obscurity as a smartphone OS.

If 2009 is too far back you can try your luck in 2012 with Firefox OS.

Joking aside, people and companies have given it a go. But a combination of bad timing and various other events never made that reality happen in our timeline.