Girls/women - whatever! - do things if it makes them feel good in the moment, later on when that good feeling has evaporated, they don’t feel obliged to follow through - unless they have an ulterior motive.
They don’t necessarily feel the need to honor a commitment or a promise as men do - unless a strong emotional connection has been established, which has the effect of conjuring up those good feelings.
When someone asks them for their phone number or email, it makes them feel good, it boosts their ego, it validates them as being desirable. Also, sometimes it’s the easiest way of getting shot of someone - they may still like you as a person, fun, funny, kind, useful for the moment etc but are not necessarily interested in you romantically.
Which one is easier, getting into a discussion with someone you are not interested in romantically to explain why and become uncomfortable, or spending 5 seconds to write down an email address, which later has the effect of boosting your ego when you receive an email?
Let’s just accept it guys, they are a lot smarter than us in this respect, have had a lot more practice in appearing sincerely interested with the smallest of efforts.
The smart thing to do is to understand their nature/psychology/motivations and turn it to our advantage…and never ever get hung up on why some girl did not reply to your email, that’s just needy behavior. Cheers.