I disagree. The browser is doing exactly what it should be- saving its state so it can be recreated upon relaunch.
Most people's phones do not have more than 16GB of storage. A few people on the cutting edge or that have too much money might have 64GB. Either way, you also have to account for pictures, music, vidoes, ebooks, apps, the OS itself, and all the other data that gets stored on these things. Sure, you could potentially create your own browser (and I encourage you to do this if you feel so strongly) that would go through the process of saving all your graphics, scripts, and HTML to disk when the OS demands you free up memory and load it all up again later when needed. You'd probably have to explain why the browser is "eating up all my phone memory" constantly. You could probably reduce the page-recreation time upon reopening that tab by a second or two compared to 4G or wifi if you're good.
Personally, I'm more than happy conserving what little space I have on my phone and waiting a whole five seconds for my pocket super computer to receive data through the goddamned ether at speeds that would have made our heads spin ten years ago.
Now, would I mind having a slider in my options to control this behavior? Not at all, I love options. I just think it would be mostly unused, which is apparently what all the browser coders thought too by not putting that in there.