What about the other way? Your boss has hired you to build a shed but by the time the project is done your building a mansion and they haven't giving you any more resources to do so. That shows that management is manipulative hiring a guy to to a job but then over time they can get job b, c and d as well for free.
That's certainly a possible explanation. Another possible explanation is that management hires people expecting them to grow and take on more duties, then ask for help when they need it. The difference is what happens when you go to management and ask for help. Your manager may honestly not know what you're doing or what you can handle. Really, it's the adult thing to do.