All zerg has to do is have a timing rhythm with their larvae injects and creep timing rhythms which are always CONSTANT 40 seconds for inject or 15 seconds for creep. When they need supply they don't need to look at their base to place the supply depot like terran instead they just hit s and then v. They don't need to tab through multiple building productions. The hardest part about zerg is KNOWING WHEN to switch from drone production to army production because terran can produce units and scv at same time where as zerg can only choose between units or workers and choosing both for zerg is inefficient. So zerg is all about constant timing patterns and a sense of when to stop droning.
Where as terran,
They have to JUGGLE a bunch of tasks of making sure all their production facilities ALWAYS have 1 unit queued up and if they que up two units for convenience they lose out on efficiency which could have been used to build another structure. This goes the same for scv production. And all the units come out at different times so its very difficult to que up 1 unit at all times when you are being harrassed. All though its okay to que up like 5 units when in endgame with boat loads of resources but terran endgame stands little chance against zerg endgame and we all know that.
I saw other things that zerg players complained was hard was that they had to replace a drone when they built a structure. Well, terran has to rally his scv back to the mineral line when its building a production unit as otherwise terran would have an idle scv when the production building is finished. And I see lot of zerg players making fun of how easy it is to call mules down. Yes its easy but if we call down mules, we can't scan you. And so we really can't get any scouting information from you if we choose to call down a mule. Where as zerg has the early overlord scouting and creep spread scouting as well as fast lings to scout. So terran has to choose to sacrafice between economy or scouting(orbital scan). Where as zerg can get both economy and scouting at the same time.
Zerg just needs to develop a timing pattern which is constant. Terran has to do a difficult juggling act.
And don't even get me started on protoss...their race is hilariously easy.
So all in all I would like to say Terran is the hardest race to macro. If you are a terran be proud of it.