This is exactly the oversimplification these posts intended to disprove. The complexity of an airplane or rocket, measured in terms of design components or interacting subsystems or what have you, is miniscule compared to any piece of software that could be written with the same budget. An even more extreme example would be science: after 6 years, the complexity of a PhD's discoveries is microscopic next to the complexity of software that he/she could have written (or did write) in the same time.
The amount of functional complexity achievable per effort invested varies dramatically between fields based on the time and cost of iteration, the time, cost, and power of relevant diagnostic tools, and the availability of domain-specific knowledge.
It's an important fact to keep in mind when judging someone else's work or deciding to switch occupations/markets/fields.
While I suspect that this is probably true. I still would like to pose the question... Are they? Or is it simply because we don't know it, it then seem easy?
Anything in the real world both incurs significant logistical cost as well as many points of unpredictable failure. I've seen thermocouples rated to 1100C catch fire when they hit 600C. Designing a new part for a new idea for a mechanical subsystem can take weeks to make, cost $1000+ easily, and might not work due to very easy to make errors.
So maybe the way to say it is that the abstraction barriers put around non-software parts of a system make them appear less complex, but what we're really saying is that the complexity inside of the box that we're dealing with is smaller.
I think what's happening here is that you know and respect the amount of effort that goes into rocketry and your mind is inferring a level of complexity that matches. Effort(times)skill is one correct way to judge the magnitude of a project, but I argue that complexity of the product bears little relation to either effort(times)skill or the project's success.
Rocketry was a bad example because the scale of a typical rocketry project is so large that dividing by cost (or effort(times)skill) becomes critical to making the comparison.
TL;DR : Sure. MechEng seems easy.
Regardless of whether a system is software or hardware: simple systems are easy, complex systems are hard.
The more complexity, points of interaction and levels of abstraction you add to either type of system the more difficult it becomes for any single person to keep track of it all.
Writing a JavaScript form validator is easy, writing a full featured bare-metal OS is hard.
Designing a simple LED light circuit is easy, designing a modern CPU is hard.