Halo =/= Smash
How would you feel if you were going against me for 10,000 bucks. We're down to one stock, and a smash ball happens to spawn right next to me, and I'm playing as bowser. Wouldn't you be upset. Items bring luck to the game (which is bad). It would work if the items spawned in the same places in a pattern at the same time but otherwise no.
You don't get it.
Let's say your smash ball spawns beside you.
First of all, the smash ball is floaty and evasive, which means you are now diverting attention to both the ball and your opponent.
Also, if you do grab the smash ball, your opponent can hit it out of you. This leads to intense mind games:
Should I rush the ball?
Should I make my way towards the ball?
Should I wait for him to get the ball?
Should I get the ball, let him take it out of me, and then take it back?
Should I get the ball and XXX
Should I get the ball and YYY
Should I let XXX get the ball at position YYY and then do ZZZ or AAA
There are so many possibilities, I wouldn't be surprised if in pro matches they just ignored it completely because they knew they would be too vulnerable.
And even if you were in a situation where you were guaranteed a final smash:
Edge hog invincibility frames > you
Jump -> Double Jump -> Up B > you
Mindgames > you
Final Smash's are just like Samus's charge shot. When are you going to use it? How are you going to use it? What position are you going to be in? Etc.
If they wanted a Final Smash = win, they would've just made the screen explode and everyone else dies.