Because each of them devices does things better than the others. The PSP was designed to play games, this is why it has buttons and input to support such.
The music player was designed to hold a lot of music.
The phone was designed to make phone calls and send/receive text messages.
The laptop was designed to be a portable computer, replacing a tower so work can be performed anywhere.
The iPhone doesn't let you edit documents effectively, you can't seriously use it as a replacement computer.
Also - if the battery is drained then that's it. When I go out to visit my girlfriend at her university I have to spend 4 hours traveling each way. If the iPod dies I move onto my DS. And so on.
You can use Youtube on the PSP through (like the iPhone) an app independent of the web browser which supports more than just Youtube as well, you can also use Flash (though not the latest version of Flash). You can't get your GPS working on your PSP? I recommend contacting Sony and telling them your
Go-Explore GPS unit is broken. Also sounds like your
PSP camera is broken.
If you can't check your stocks then I suggest you update your PSP firmware so the web browser can work.
iTunes not working? You could try using
Missing Sync if you really need it.
I'm not sure if you're being serious or just joking about.