The simple answer is humans make errors. Even mailing a letter the post office often applies corrections to written addresses which contain mistakes. This is only possible because the address system is relatively sparse and over-constrained.
For example:
Bob Smith
162 Portsmouth St
Denver, CO 42348
Encodes the city in both the zip and city / state, allowing either to be wrong. If 162 doesn't exist on that street, it's possible they can correct it based on the name. If it's Portsmouth Ave not St, they likely can figure that out.
None of that flexibility exists in URLs. A single mistyped character or misspelling and you are going to the wrong place with no way of getting where you want (without search engines).
It's a system which works very well for machines, and only passably well for error-prone humans.
This is a somewhat compelling reason why people don't use URLs like they do addresses (that plus the fact that physical locations are "discoverable", so if you know something is in the general vicinity of 40th street you can probably wander around a bit to find it).
But I don't know if it explains why people don't understand URLs. Physical location addresses are no more complicated than URLs (with the exception of URL-encoded blobs in URLs, which are not usually human-readable). That said, I don't usually talk about this stuff with non-tech types, so maybe the average person does understand URLs more or less.
For example:
Bob Smith 162 Portsmouth St Denver, CO 42348
Encodes the city in both the zip and city / state, allowing either to be wrong. If 162 doesn't exist on that street, it's possible they can correct it based on the name. If it's Portsmouth Ave not St, they likely can figure that out.
None of that flexibility exists in URLs. A single mistyped character or misspelling and you are going to the wrong place with no way of getting where you want (without search engines).
It's a system which works very well for machines, and only passably well for error-prone humans.