It probably just confuse more people if there is a lock button or feature on a web browser, and might make the browser use up more space on the hard drive.
You can always mouse right click on another tab, or an empty space on your tab bar and cllick Reopen/restore closed tab on the rigjht click menu,
In Firefox, it is undo closed tab.
You can go into your history, and re-open your closed tab by creating a new tab, and clicking on the website address in your history.
sometimes though closing the tab can erase everything on that tab like text or choices made. Should be but a simple code to put in that allows you to right click on a tab and lock it into place.
I'm guesing Firefox, Chrome, IE, and other browser makers just want to make sure their browser works, and let the add-on makers make customization add-on for their browsers.
Opera has that feature, it is simply allows you to "pin" tabs and they will stay open regardless of whether you have massacred a bunch of open tabs, or your browser crashed or you [accidentally or intentionally] closed it yourself.
