Because your phone and iPad have tiny little antennas and very low-powered receivers. The placement of your hands on the device affects how they receive signals as well. Your PC is not encumbered by any of that. Big antenna(s), lots of power and you aren't bear-hugging it while it works. Wrong password or wrong security settings will prevent it from connecting.
It has nothing to do with incorrect passwords. And it is not security settings because when the other devices picks up the signal perfectly...when coloser to the signals, the computer locks without issue. What else? Or bug fixes?
Wifi is incorrect band or channel for the pc...i have this on a laptop.it can see the N band router but as its a G band wifi carded pc it cannot connect...took me a while to remember it was a old laptop..check your routers configuration to for the security settings i like things set to auto...check the encription method on the router...or as harryberlin said diffrent wifi card.somtimes its not worth fighting with pc parts and reliebility when ebay is a few clicks away..speaking from experience of being a stubbon tightass