Partly, It might be that not every key suits everyone's voice. Also you get variations in arrangements of the same song. It'd all be a bit dull if we all played exactly the same way. It would probably be a good thing to include I IV V7 etc, as guidelines, but if you can read the bloody things that fast, you probably don't need them! Probably best thing to do is post a request for chords of a particular song if you're stuck. Somebody out there will know it!

Have fun...

English Jon