It's not that girls take it seriously, it's that when you send someone a message or call them, you expect them to call/message you back in a timely fashion.
For example, if you left a girl a voicemail, wouldn't you be a little annoyed if she just never called you back? And wouldn't you begin to question why she never got back to you? Is she just rude, does she not like talking to you, does she think you're irritating, did she not get the voicemail? That's how girls think.
We'll send a guy a text in order to communicate efficiently and let them know that we don't expect a long convo from them, and we'll be pleased by our wittiness for about 5 minutes. After that, if we haven't heard back, we start to wonder why not. Most girls immediately think the guy is avoiding her (because boys do this regularly), rather than assuming he's busy/in the shower/whatever.
My recommendation is to always return calls and texts from girls you like in a reasonable amount of time, otherwise they will begin to think that you're uninterested or a d-bag, regardless of what you say to them when you do talk.