Zerg is designed to have better economy and production than the other two races (through larva mechanics). So for Terran and Protoss to beat zerg, they have to deal meaningful early game damage, otherwise, zerg will be ahead in economy and units, which eventually snowballs into a victory for them. Even at max supply, I can't argue that Terran and Protoss have better compositions than zerg (zerg has strong tier 3 units like viper, swarmhost, lurkers).
With the current meta already figured out, all possible early game harassment strategies against zerg are figured out and can be defended. What this means is that, top zergs these days will come out on top into the mid game and snowball into victory unless they make a big mistake from defending the early game harassments (the top ones pretty much don't). This also means that even at the highest skill level, a Terran or Protoss unfortunately still has to rely on Zerg to make mistakes in order to win - just executing your strategy close to perfection does not lead to victory since you will be behind if zerg defends it properly (which is likely since meta is figured out). This is why top zergs like Reynor who has fantastic mechanics and have memorized the protoss and terran playbook have a close to 100% (I remember its 90+%) win-rate against protoss for example.
In fact, diving deeper into this, even with new patches that changes up the dynamics and introduces new harassment timings etc, there eventually must come to a point where zerg must be able to defend every possible harassment, otherwise terran / protoss who play perfectly can keep abusing the strategy and always win out, which I'm sure Blizzard won't allow to happen. Blizzard also unlikely will allow the existance of a strategy that can only be defended non-deterministically (based on luck). So if the eventual state of stability for the game is such that every strategy can be defended by zerg in some deterministic way, then zerg will always have the advantage against the other two races via the observation stated above. Anyway, just theorycrafting, would love to get some feedback.