Albrecht Dürer would love to hear your theory on roman characters violating their neighbors. And by the way the Romans were known to violate their neighbors quite often. As you can see from the photo that the Roman stone carvers were not to concern about their spacing as the modern world is. Kerning started when type was invented and that is also how the word leading came to be used. I hate to challenge you on this Bob because usually you have some great insights but when Motzart was told he should only write operas in Italian we would never be able to enjoy The Magic Flute.
With the advent of moveable type, around 1450ce, there evolved rules for typography. Actually guidelines rather than rules. One rule that was inherent in the physical nature of moveable type was the concept of the inviolability of the type body.
Since each character was cast on a rectangular slug, a single character was bound by that rectangle. No part of any character could extend beyond its bounding rectangle**. That being the case, no part of any character could occupy any part of any neighboring character's bounding rectangle. This was not merely an arbitrary rule, it was physically impossible.
Hence the development of ligatures.
Since the coming of soft type there is no type body and thus nothing to violate. But...for the past ~600 years, which span contains virtually all of the time that printed material has been universally distributed and seen, the sovereignty of the type body has been the physical reality and has become the visual norm. Improper typography is not usually apparent as such to most people, it just doesn't look right and subconsciously makes them uncomfortable.
**There are certain specimen type faces that violate this with irregularly shaped slugs. Generally these encroachments are decorative extensions like long tails on 'Q's etc. Moreover, creative type setters have been known to file, notch, and otherwise modify slugs for various special purposes.