Because two successes is usually considered better than one. Isn't that the point behind two effective actions?
You're still just getting two shifts/FP. Those shifts don't become better because they're attached to multiple actions.
Simple math.
Lets say I have a skill of 4, a stunt that grants plus one, and the weapon value would be 2.
Assume neutral rolls for attack and defense, assume defender has the same skill.
each "attack", though this could apply to almost anything that is opposed, would deal three shifts of effect. 6 shifts of success. Twice as much as 3 shifts of success.
Now, lets assume I can use three aspects for effect on this "attack" that's +6 (+2 per aspect/fate point used in this fashion). I cannot use each aspect more than once threfore I can only get three tags/invokes for this roll.
Wait! I get two attacks so I can spend six fate points this exchange. That means double the shifts of success in one exchange.
+6 becomes +12 overall.
So, wait.
The problem is that multiple actions raise the cap on spending per exchange? That's it?
You can get the exact same effect by taking different Aspects on your character! And Aspects are free!
Also, using Flurry as written you get zero shifts of success for two attacks in your example. Because both attacks miss.
Grapples:
Normally a grappled foe only gets one chance (one exchange) to remove the aspect "grappled" before that foe is good and screwed. Grapples are very powerful.
With multiple actions grapple become far weaker, since multiple actions per exchange can grant multiple chances to remove the grappled aspect.
Grapples would then need a upgrade, upgrade stunts, or a rewrite. just to make multiple actions work. The system was designed not to have them...yet here some people are trying to force them in.
You know what else increases the chance to beat a grapple? +1 to a skill. And that's totally legal. As long as multiple actions don't give a disproportionate-to-cost increase in grapple-beating capability, they're no better.
Let's compare probabilities.
If I'm facing a Good Grapple with my Good skill, I have a 61.7% chance to escape. With a +1 bonus, I have an 81.5% chance.
If I instead use a two action flurry, I have two 18.5% chances. Worse than +1, which costs one fourth the stress. In fact, it's worse than doing nothing.
I think the math is on my side here.
If you're still not convinced, I propose a test fight. Two dudes with armour 2, weapon 3 + 2 stunt, mental/physical stress track 4, 4 FP, and Weapons 4. One guy gets True Aim. The other gets the ability to use Flurry for free whenever he feels like it, or maybe just Supernatural Martial Arts.
I expect True Aim guy to win.