The three races are different....
And even if they weren't, your analogy is incorrect. Zerg can inject multiple hatcheries at once and protoss can chrono multiple Nexuses (Nexi?) at once. Thus, terran should be able to drop multiple mules at once. Based on this analogy, however, one could argue that terran should be limited to 1 mule per orbital command at any given time.
But even this analogy is not persuasive because the three races are different.
Make it one mule per cc then , or give zerg auto inject, wouldnt effect early game and it would help zerg late game , this game is supposed to be RTS let us focus on the strategy , not real time larvae inject. Zerg HAVE to inject every 40 secs both toss and terran can wait and build up energy and then dump it , zerg cant make up for the delay ......
If you want a less real time and a more strategy based game, while still being within the RTS genre, try the Total War series. I like those games, but they involve a lot less micro/mechanics/macro. Your strategy decisions in that series tend to be turn based, with optional real time combat (never as micro intensive as SC2 though).
Auto injects would be too powerful an ability for lower level zergs. These lower league players with auto injects could easily jump up a league or so. Injects, supply blocks, and floating money are some of the very common reasons why people are in the lower leagues. Changing this would make zerg the easiest race to play. Unlike protoss and terran, we have the easiest production mechanic in the game. We can quickly mass units through our hotkeyed hatcheries and we don't get punished for not queuing our units on time. Our larvae mechanic gives us some leeway in not producing units all the time. Compare that to terran where if your barracks stop producing due to an unintentional mistake, you will end up with too few units.
The inject mechanic is challenging, but it is no more challenging than remembering to cycle through your barracks, factories, and starports to build the units you need. If you have an idle production facility as terran, you are usually hurting yourself. With zerg, you can just press your hatchery hotkey, see the larvae in all your hatcheries and produce your units all at the same time. Larvae stack to around 20 (could be off slightly) and your natural hatchery larvae production continues till you hit 3. Simply put, you don't have to constantly go through your production facilities making units.
Zergs not injecting is like terrans not making units constantly. Terrans cannot afford to queue units when playing efficiently. Either queuing units or not being exact in your train new unit timings sets a terran back. Similarly, not injecting on time sets a zerg back. Both races are different, thus while there challenges are not the same, they both have challenges.
Note that I am a zerg player. I rarely play any race other than zerg. Even I know that auto inject is way too powerful, unless you create auto production for terrans (e.g. queue units without spending minerals till they start training). While these changes might sound good to you, they would destroy what makes SC2 unique. SC2 is not a pure strategy game. It is a strategy game that emphasizes the real time aspect. I would hate to see that change, and I would hate to see Blizzard make such a change to the game.