The problem is that sadly, closing a tab is often easier in practice than using the "back" functionality:
- many websites are poorly coded and break the "back" functionality, making it (sadly) unreliable. On pages that load content dynamically, results are not always predictable either
- my left hand is already positioned in such a way that hitting cmd/ctrl + w is trivial, whereas going back requires reaching all the way to the backspace button/the mouse/doing a multi-finger gesture on the trackpad
- if you had to submit content to get to the current page, usually going back will trigger a "confirm form resubmission" warning
- on mobile devices, the "back" functionality is not very prominently displayed. On Chrome, you have to click the "more" icon to access it
In addition, manually opening in a new tab (which requires right-clicking, middle-clicking, or ctrl/cmd + click) is not always as trivial as people imply:
- people who are using a trackpad do not have a middle button
- right-clicking is has not been part of Apple's design for a long time. You can perform them using cmd + click or using two fingers on the trackpad (I believe it is disabled by default), but it's not as convenient
- On a MacBook, the modifier keys (fn/control/option/command) are all placed next to each other and it's not always obvious which one does what. It's easy to get confused or to accidentally hit the wrong one. I've personally done option + click by mistake countless times, which opens a prompt to save the link, and it's always pretty frustrating
- the right-click equivalent on touch screens (long press) is a bit clunky
And this is coming from a tech-savvy user who actually knows about these options -- you'd be surprised at how many people don't. Of course, having the ability to make the decision is important for power users, but I can't help but wonder if for less sophisticated users the practice is not so bad after all. The usability studies I could find are pretty dated and I would not be surprised that usage patterns have changed.