When you open a new tab, the original tab continues to run in the background, but you can't do that for your history.
Imagine a page which shows a number that increments every second the page is loaded. When you open a new tab, it continues to increment. When you go back, should it continue from where you left it, or behave as if it was loaded the whole time? The answer will be different for different sites, and the browser's default behaviour will sometimes be wrong.
Imagine a page which shows a number that increments every second the page is loaded. When you open a new tab, it continues to increment. When you go back, should it continue from where you left it, or behave as if it was loaded the whole time? The answer will be different for different sites, and the browser's default behaviour will sometimes be wrong.