Again, you're not wrong for wanting a phone with a bigger battery. We all want that too.
But we also have to realize that smartphone makers are making these things to sell. To consumers, something that is sleek and catches the eye is going to be more appealing than a chunky device that lasts more than 3 days on a single charge.
We will get there someday, having a big battery in a sleek device once those battery balls are implemented. But until then, don't expect much change.
It's also kinda funny that I'm saying this because the S9 is 8.5mm thick. It has actually gotten thicker and thicker.