Kickers have to sometimes tackle, run fakes, kick onsides, deal with mishandled snaps or holds, and adjust for the conditions, like wind or how the opponent is trying to bother the kick. Plus the kick is from different parts of the field, unlike a free throw. But it doesn't matter, since someone taking a free throw has to be already in the game playing. There's no designated FT shooter.
At some point, being able to score 3 points from unlimited range just breaks the game of football to the degree that an increased risk of mishandling a bad snap/hold doesn't really matter, and you won't ever need to kick onside, either.
