Another reason for having two EQ's and two compressors is that some models only come in mono. But if you want to master using them you'll need to apply them to both channels.
But the big gotchya on doing that is, of course, that you have to set each 'side' individually. This isn't too hard usually with an eq -- esp if it's got fairly well calibrated controls so that you can save your ears for fine tuning the settings.
But running two separate compressors on the two sides of a stereo pair is very, very tricky. And that's why most folks use two channel 'stereo' compressors which typically have a "stereo ganging" function that yokes one side to the other. Even past getting the proper settings this prevents a loud sound on one side from creating a very different compression profile as the sounds shift through the track -- which in the mix can produce odd stereo "breathing" and stereo sound stage shift.
This last bit above applies to plug ins just as much as analog units.
When you're compressing a stereo signal it's usually best to use a compressor in "ganged" mode. (And of course many EQ plug ins and outboard units also come with a stereo function.)