leblanc burst is too high for the type of safe assassin she is.
Damage potential isn't counterbalanced by speed or safety. I mean really - look at any assassin in the game. Speed is not factoring into that damage.
Damage isn't really hit by defense or range either.
And with defense, mobility, and range all being factors to allow a champion to survive long enough to do their job: there is no shortage of champions in LoL that can survive some perilous situations while delivering some serious pain.
Leblanc's single-target DPS being rather high is more a case of balancing single-target DPS vs. AoE damage, or dueling power vs. team-fight power.
And LeBlanc's team fight presence is awful.
Look at any marksman, mage, assassin, or fighter and you'll pretty much always see the tools necessary to take out a solid 50% of the enemy team's collective HP. Most can hit that mark in a standard game.
All Marksman can get their with auto-attacks alone.
The AoE on Mages allows a well-placed combo to hit that mark on the vast majority of those.
Assassins often have re-set mechanics to turn 1 kill into 2, and generally have strong auto-attacks to finish the job.
Fighters tend to have a combination of the above 3 mechanics: relying on strong auto-attacks, AoE effects, and resets to hit a 50% mark.
LeBlanc has weak marks in all 3 of those mechanics. She has decent AoE damage but it's very high risk to utilize that in damaging multiple opponents, her auto-attacks are beyond wet-noodle weak, and she has no resets on a combo with CDs that don't really allow more than 1 rotation in a typical fight.
Any scenario where LeBlanc can realistically hit a 50% mark pretty much everyone else could hit a 75%~100% mark and have a good shot at a Pentakill.
LeBlanc's damage output isn't exactly high or unbalanced, as it's just all single-target burst.
Not super fun to play against, but it's not to high given what LeBlanc realistically needs to do to win. She effectively needs more gold and kills to push for a win than just about anyone else, in turn being one of the least efficient champions in the game when given gold or kills.