Multitasking, mostly doesn't exist. Even for starcraft pros. What looks like multitasking is single focus shifting in very high frequency, back and forth, with very high accuracy, relevance and efficiency, between most relevant areas of available priority work.
For untrained newbies, the difference isn't that they suck at multitasking. It's this:
1) their frequency, in terms of shifting back and forth between varied targets of available priority work, one by one, is lower in general,
2) their capacity to maintain balance and clarity, while going high frequency - in terms of shifting between varied targets of available priority work, one by one, is much lower (maintaining balance and clarity, i.e. sense of order and keeping track of what they're doing, aligning each task to fit into the build they're trying to execute etc., not missing basic actions such as blocking the ramp with zealots etc.)
Single-tasking exists and it doesn't work, not in terms of how most people view it, and certainly not in starcraft. The primary reason is obvious, downtime. Most tasks have durations of time where further work is irrelevant, inefficient, or not possible, for these durations.
Let's say you're managing your scouting scv - after you've glanced at it's environment in the opponent's base, there's downtime. This tiny flash of time is enough to secure a glimpse and mentally update status of the situation.
More miliseconds on this area of work becomes irrelevant and inefficient - only for a few more miliseconds or seconds, depending on the prior intel and scouting scv's location - but it does. That's because in that brief window of time, you can shift focus to another area and perform a tiny portion of work that adds more value to your build, and your ability to keep executing and adapting your build accurately.
For example, send worker to an expansion's location, check for holes in the defenses on the ramp, adjust marines formation, send scv to the location of the next supply depot, then come back to check in on the scouting scv. That adds more strength to the build than to just watch the scouting scv, when you've already re-routed it and ensured it won't die for the next few seconds.
The following terms offer a more accurate representation of how doing more, in less time, looks like and works.
Available priority work - whatever is priority but also can be worked on right this second, in favor of another priority that's been taken care of for some miliseconds or seconds.
Example - scout can be a priority task but you don't just keep watching the scouting probe. Instead, you keep shifting your attention back and forth between base, minimap, scouting probe's environment, in adaptation to what's these acts of attention reveal.
Singular focus shifting in high frequency - singular as in, not scattered, one task at a time. When this focus becomes trained in terms of shifting, back and forth, between available priority work areas, it can look like multitasking. In reality, it's utilizing all kinds of downtime periods, from the tiniest flashes of miliseconds to longer durations. In doing so, it shifts to a tiny, or larger, bit of another priority work that can be completed in that very small fragment of time.
Over time, of course, this builds up to a huge difference. Both in terms of being highly aware and executing with great responsiveness, as in:
1) being rock solid and able to pre-emptively re-check things, keep track of things, update mental status of how the build's going, be extremely alert for any problems or threats that might occur, mentally noting scout intel, checking no holes, checking no build errors etc.,
2) being extremely responsive and immediate to opponent's moves, to one's own actions and errors in terms of build, and in being on time with the right actions where they're needed for the build, army management, scouting management, workers etc.
Essentially, what skill is, what a pro does, in part, is that through so much training, a very sophisticated kind of attention develops. It's the kind of attention that can operate with extremely high frequency, rapidly shifting between areas of feasible priority work, while maintaing full clarity and sense of order.
This enables constantly completing volumes of extra work on the microscopic scale (e.g. glancing at a minimap, checking the supply, sending worker to an expansion location, sending a ling to scout - most of these take miliseconds) but in a way that tends to fit each of these tasks such that they all add up to an integrated, solid build - one that tends to accomplish more, more securely, in less time.
This is an aspect of how someone with great skills, a progamer, ends up gaining strategic advantages over the opponent, in terms of proper adaptation, responsiveness, making much less errors, being more aware of opportunities and threats, executing more strategically relevant, and accurate, actions per unit of time, and so on.