Something I’ve seen a lot of on the internet;
“Why couldn’t she have been polite and message me back that she wasn’t interested?”
This can also be extrapolated to Real Life with calls or texts, and some such statements are filled with more bitter, entitled misogyny than others, but it all boils down to this: “Why didn’t this person reject me to my face?”
Why would you want to be rejected to your face? I don’t want to be rejected to my face! When I see the little pink letter on my the top of my OK Cupid page, I want to see a message from someone who thinks I’m pretty and groovy and wants my bod. I get a little excited when I see that little icon, even if I know that the person messaging me is probably illiterate and/or someone I’m not attracted to. I don’t want to have that little frisson of excitement and then read a rejection! A rejection message would bum me out, while no response at all will probably not even register as a blip on my radar since I have a policy on forgetting about guys right after I message them.
Same goes for more real-life non-serious dating-type things. A slow fade is effective and relatively painless way to let someone know you don’t want to take things further. “Oh, I see you are not responding to me! This is the behavior of someone who is Not That Into Me and my cue to push them out of mind and move on! Done and done.” I don’t need someone to tell me that they’re not interested in me, and I certainly don’t need a list of my character flaws as a parting gift.
Obviously the rules change after you get to Teh Sexytimes or otherwise raise the stakes, but in the low-stakes preliminary period why make a big production out of “eh, not that into you”?