Let's talk about the imperforate 1851 issues. Perforations were not in use yet and these stamps were cut apart by scissors. Since there were no perforations, the stamp designs were intentionally placed close together to reduce the amount of paper needed to print the stamps, hence, reducing costs. These are Scott numbers 5 to 17.
Scott numbers 1 and 2 were spaced far enough apart on the printing plate that copies with nice wide margins are relatively easy to find. Scott numbers 5 to 17 are difficult to find with wide margins for two reasons: the close placement of the stamp designs on the printing plate and the care (of lack thereof) of the person cutting the stamps apart. A tiny slip of the scissors turned a VF copy into an AVE or F copy.
The 1857 perforated issues, Scott numbers 18 to 39 are equally difficult to find with nice margins. These stamps were perforated 15 ½, making larger multiples very fragile. Most multiples have hinge reinforced perforations to keep them intact. Again, nicely centered copies with wide margins are difficult to find for two reasons: the narrow spacing of the stamp designs again and secondly, this was the first attempt at perforating stamps so the tolerance for off center stamps was pretty large.
Today, stamp designs are widely spaced apart so that VF copies are quite the norm. But 160 years ago, stamp designs were place much closer together making nicely centered stamps difficult to find.