The reason they make everything so reliant on coin flips is for their idea of balance. In a game where the base cost for an effect is zero, you need to tack on additional costs. Discarding energy in play, or cards in hand, or milling yourself, or even letting your opponent draw a prize is considered a cost. Flipping coins is a way around this, but with each flip you do, you lessen the overall chance of performing the effect.
One flip: 50% chance of winning
Two: 25% chance
Three 12.5% chance
Four: 6.25% chance
And so on and so forth. It's an extremely lazy way of costing things. Oak has the most powerful effect in the game, the dreaded "Draw 7" effect, and is balanced by discarding your hand. Elm has a Draw 7 effect, but is too steeply costed (namely, being unable to play any more trainers) to truly shine. Birch doesn't even have a Draw 7 effect.
It does, however, require a lot of playtesting to correctly balance the costs of a card, and you need to playtest extensively with each revision. How does it work in multiples? How does it work in the early game? The late game? How does it interact with the other cards in the deck? For that matter, is it weak in some decks but overly powerful in others? These and more are questions that the developers have to answer to their satisfaction, and it's easier to simply put a coin-flip effect and neuter the card's chance of being used seriously, than to playtest five dozen times (literally) per revision.