It's all a matter of strategy.
For example, bronze medalist on Vault, Giulia Steingruber, has one 6.2 D vault and another 5.8 D vault. Dipa Karmakar from India performs a 6.0 D vault and a 7.0 D vault (with a fall). Even if you count the fall, she would still have the same difficulty as Steingruber. In the end, execution matters the most, because those who had to count a fall didn't manage to earn a medal.
I totally agree with you that a fall should be more deducted, though. I think two points off of the final score would be enough.