If you can remember different first names, why couldn't you remember different last names?
As for kids: it's pretty simple, pick a name of one of the parents. Nothing confusing about it. A name (first+last combo) is an identifier of a person.
Are you seriously baffled why people choose to do symbolic things to show their love for each other and show how they are bringing their lives together, even when it isn't convenient to do so?
If you genuinely are then I can't imagine how you are living day-to-day in human society because people do all kind of things like that in all parts of society.
It's not a constructive expectation of a society to have imho. A name is an identifier of a person. You need a first and last one to have less same-name clashes.
What portion of people do you think have any of those other than the email address (which for many people's personal addresses doesn't even include their full name)? For my wife at least, the email address is what convinced her to change her name because it made it possible for her to get the address [email protected].