I believe Tobey McQuire declined to do Spider-Man 4 once Sam Raimi was no longer involved. For Sam's part, he wanted a higher film budget than Sony was willing to offer. And then Sony wanted a picture finished by a date that he didn't believe he could meet.
And for Sony's part, their license to produce movies based on the Spider-Man property was probably about to expire if they didn't produce something to keep it active.
I think rebooting Spider-Man is a healthy thing, personally. How many different versions/interpretations of him have appeared in the comics? Probably several, right?
I don't read superhero comics, however I have a very dim recollection of one of the cartoon shows from probably the late '70s. And the theme song I remember from it is NOT the same one everybody else remembers. I also don't recall Peter Parker being anywhere near as dorky as Tobey McQuire makes him out to be (though he definitely wasn't popular by any stretch).
My point is, Sam Raimi's movies represent just one possible interpretation of the Spider-Man world. James Cameron's pitch might have represented another. For whatever reason, Raimi's interpretation is the one he decided moviegoing audiences should see and become familiar with. Because it's what he saw when he looked at the comics, cartoons, or whatever else he drew inspiration from. Good for him.
Except he's not directing anymore. He's not even executive-producing.
Do we pick a director-for-hire to take his place, write a script for him, and tell him to just copy the style of Sam Raimi's movies just so 'most people' won't even suspect Raimi's absence? What kind of director would take on such a thankless task, knowing he's just going to receive flak for it afterwards? Maybe Brett Ratner would agree to it. People love giving him flak.
How many movies can most franchises sustain anyway before needing to be refreshed? I'm thinking it's usually three at the most. Something always seems to happen by the fourth film if not sooner. Either the director leaves, or the actor, or the studio starts asserting more control and demanding tighter budgets... and somehow everyone just 'seems to know' that it's run out of steam. Audiences stop taking you seriously once the number in your movie's title rolls up that high. With Star Trek it happened at the number 'V'.
Meanwhile, there still remain at least dozens of other possible interpretations of the property not yet explored. And if Sony is going to produce more movies anyway...
So, to the question "why reboot a franchise"? Why can't "Why the hell not?" be a more-than-justifiable answer?