Everything from 4 rax to macro is scouted by overlords and can easily be prepared for, while zerg cheese is abundant, can come at any stage in the game and instantly wins if you didn't do everything right (just for reference, if you proxy gate with stalkers against proxy rax, you can lose 2 stalkers for free and still stomp just with decent micro).
Zerg has enough to defend at all points in the game, naturally, so the only problem is whether or not they scout... which you simply can't deny. It's not even realistic to consistently deny one slow overlord from seeing your build, and so making zerg automatically ahead.
You need to kill 20 workers just to barely survive a dedicated 8+ minute roach ravager push as mech. (No wonder maru lost 3 games to the same timing push in a GSL finals.) There should never be a scenario where terran can win the game before 10 minutes because roaches stomp EVERYTHING on ground no matter what, and any decent zerg can just make them after the push is there while microing the drones away, so your damage won't be big even if successful.
Hellions also do nothing because they always make an evo chamber between the mineral line and their hatchery, so losing a substantial amount of workers is impossible and blue flame opener is not good due to this, BC opener is even worse since you are not just vulnerable to a roach corruptor timing attack but also mutas are guaranteed to deal a ton of damage while shutting down the potential of your BCs.
2 base muta is an auto win every single time against mech because YOU. CAN'T. KILL. THEM. because there is no point in the game (even if you go 2 factory opener) where your cyclones can go out on the map and beat the mutas.
Mech auto loses to amazing outplays such as locusts landing on tanks, not paying attention for 0.5 seconds to what is behind your tanks and getting neural'd, corrosive bile preventing pushes, lings getting to your tanks after zerg instantly re-maxes, and literally being able to tech switch at any time into air when you only have 2 thors, automatically losing the game, among other things. (just as a reminder there have been games where zerg loses 5 armies and still just wins). Bio auto loses to fungal, burrowed banelings, not dealing damage (which happens all the time against anyone decent due to obviously overlords but also queens and spores, there are NO opportunities to deal damage if the zerg is good) and easily getting overwhelmed, hold position lurkers, being physically unable to deny expansions because roaches can always overwhelm you etc.
Protoss is much worse. If a protoss is ever touched by a terran, the game is being played at a subpar level by the protoss player at best.
2 base pushes are comically weak and if you don't see that then you should be playing the corny chinese 4 button game - 7+ minute pushes get stomped on 3 base, 2 base pushes get obliterated by blink into colossus or counter attacking while a-moving chargelots (even with 0 colossi).
There will never be an equivalent to making shield battery colossus on 2 base against a terran doing a 2 base push. Because if protoss does a 3 base blink attack? Oh that's right, you can produce at maximum capacity off of 1-1-1 and you will lose if they micro well unless you have every possible defense ready. There will also never be an equivalent to colossus archon chargelot a-move which is mathematically better than both mech and bio and gets out much sooner than tank thor hellbat does.
Alright, so you have to play out a long, tedious, wrist-breaking macro game, being so good at the game you can micro, position, stim, siege, make it so medivacs aren't too far forward, split, concave, notice exactly where their high templars are and EMP there unless you want to lose the game to one spell, know when to pull back, and all of that just for one fight. Then you have to deal with tempests, DT counter attacks, disruptors (a very skilled unit that utterly outplays your noob loser opponent by pressing 1 button while they have to react like a pro and even then still get efficiency) etc.
This is assuming you've gotten past the phase of the game where you build up tech until you can survive against a colossus chargelot a-move.
Not even mentioning cannon rush (or other DISPROPORTIONATELY strong rushes compared to anything terran can do), which is an auto win every time unless terran somehow survives and goes into a long game where they are behind, then comes back. But otherwise there is quite literally zero counterplay, their cannon production is obscenely faster than how fast you can make marines and a bunker. It also wins against proxy rax since they can effortlessly afford to make 2 cannons at home by the time 4 marines arrive. zest vs ryung is an example, too bad zest took his hands off his keyboard after cannon rushing because it's that stupidly OP.
Not to mention blink and pheonix give protoss complete map control.
Not even mentioning observers: a free map hack.
And remember back when it was shown 48% of EU GMs were tosses? Remember heromarine streams where literally everyone he plays against is a toss? BUT TOSS SO UNDERPOWERED! NO GSL WINS BY PLAYERS WHO LOSE 17 PROBES TO A LIBERATOR!!!!!! (And that is now false too since hero obliterated maru while playing worse.) When will game devs realize that making something weak and something else completely overpowered just means people will pick the OP thing and then will play the abusive easy OP thing vs the abusive easy OP thing (or another), meaning no fun for anyone?