Going first when you win. This is the biggest offender. Going first in Koi-koi is a huge advantage. Hitting a double match on a full board of eight cards gives you a huge starting lead that significantly disadvantages your opponent. Suddenly instead of being able to match against 7 cards they can only match against 6. Under these conditions, when compared to going first the second player has only 75% of the matching options. And hitting a double match is statistically most common on the first move of the game so it happens a fair bit. When you alternate back and forth this advantage is evenly spread. When the winner goes first, it creates a huge incentive to pretty much never call Koi-kio unless it's late in the game and youre losing. This is even more true when you consider the second default rule at play.
Starting multipliers. Out of the gate, a starting multiplier further disincentivizes the risk of calling koi-kio. When even the weakest 1 point Yaku can score you three or four points off of this multiplier system there really isn't a good reason to risk it, especially when there's such a powerful advantage to going first.
The two combined mean that there are very very few situations where you need to even bother to think about the risk vs reward side of the game. I've read that the rules are designed to encourage aggressive play but they have the opposite effect. They encourage low-risk fast-play that often devolves into an unbeatable snowball effect for who ever hits the first yaku.
Try it in your next game. Shoot for the fastest yaku you can hit and never call koi-koi. If you hit the first yaku and your board state control skill is roughly in the same ballpark as your opponent you'll rarely lose.