Okay, so then why am I not simply creating that widget myself and selling it for $1000 on my own, if it's that simple? Likely because of both of the following, to name a few:

1. I don't have access to the raw materials and tools needed to create it, they were provided by my employer
2. I haven't spent the money on the marketing that would make customers willing to pay me $1000 for that widget (see: Apple infamously, and largely successfully, charging significantly more for products than comparable equivalent products of other brands)

The bottom line is, there is much more than "your work" that goes into that widget ultimately being successfully sold for $1000, and that's why you can't accomplish the same on your own (or else you would, right?), and it's why you do not have a rightful claim to that $920.

Now, if you're able to create and sell that widget for any more than the same $80 of time/resources, then you should go into business for yourself, and can reasonably say that the business offering you only $80 was ripping you off.

But in the real world, that's rarely the case for very long. Those situations are extremely rare, and when they're discovered by individuals, they're capitalized on (pardoned the pun) pretty much instantly--all it takes is one person to 'close the gap', generally speaking.