Humans learn social skills through osmosis and modelling. Knowing some cool guys and seeing how they carry themselves around women can go a long way.Only when people fail to realize that having conversations with people is an inherently normal thing to do and act with social ineptitude.
A large part of the problem is that people doing this are people who would have problems stopping someone on the street to ask for directions because they have no ability to act normally in a social setting, so of course when they attempt this it comes off as awkward, weird and makes the other person feel uncomfortable.
Plus the added pressure they put on themselves to "get a result" leads to nervousness and non-normal behavior.
Basically it's weird and unnatural many times because the person attempting it is socially inept and because on top of this they have added pressure of being very outcome dependant which increases their nervousness and awkwardness.
For a person comfortable with being social ot doesn't even come off as being a PUA attempt, it simply comes off as a normal interaction and conversation with another person.