Your argument is a two edged sword because it is also stupid and unrealistic to expect the giant to stand there motionless waiting for the pixies to kill it - this is more often what you see in FE combat.
If the combat was happening in real-time then both combatants would move at the same time, but the pixie would stab with his dagger three times in three seconds, while the giant would bash his club once in the same three seconds(because the pixie is three times faster). It goes like this:
1st second: pixie stabs the giant. Giant is lifting his club above his head.
2nd second: pixie stabs the giant again. Giant now has the club over his head and starts to bring it down on the pixie.
3rd second: pixie stabs the giant 3rd time. Giant finally brings the club down and bashes the pixie(both happening roughly at the same time).
The giant did not just stand there motionless waiting for the pixie to kill it, he was just acting slower.
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But the game is turn-based, not real-time. You have to somehow translate the 'real' combat above into a turn-based system so that the result of that fight would be the same. So you make one second of real time(roughly) equal to one turn. Remember the combat needs to have the same result, we are changing the game system(from realtime to turnbased), not the fight outcome.
1st turn of turn based(=1st second of realtime) = pixie stabs the giant.
2nd turn of turn based(=2nd second of realtime) = pixie stabs the giant.
3rd turn of turn based(=3rd second of realtime) = pixie stabs the giant.
4th turn of turn based(= the same 3rd second of real time - both attacked at the 3 sec mark) = giant bashes the pixie.
Overall result is the same as in the real-time fight. Again, the giant is not 'standing there motionless', he is assumed to bring his club into attack position during turns 1 and 2, however in a turn-based system the actual figure of the giant does not visually change when it's not his turn, instead what he did in turns 1 and 2 is played as a seamless animation when he finally gets to act.
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Now, what would happen if we made the combatants trade blow for blow like you suggest we should?
1st turn of turn based= pixie stabs the giant.
2nd turn of turn based = giant bashes the pixie.
3rd turn of turn based = pixie stabs the giant.
4th turn of turn based = giant bashes the pixie.
You just made the pixie stab 2 times instead of 3, and the giant bash 2 times instead of 1. In other words, you reduced the pixie's damage output by 33%, and increased the giant's damage output by 100%. That most certainly is not the same fight outcome as the real-time fight we were trying to translate into turn-based rules.
(and yes, i have way too much free time on my hands 
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