Because it is the other 90% of playing guard. Closing your legs is very effective at trapping your opponent, but almost everything needed to attack your opponent happens when you open those legs. So you may as well get used to it! Also, just doing your best to show how strong your legs are will eventually get you opened, and then, guess what? you will be forced to play open guard.
For many beginners the very idea of opening the guard is a disappointing one, even a bit scary. It can seem like a direct route to letting your opponent take side position. I'm going to tell you exactly why. First picture the scene. You've got your two-hundred and sixty lb buddy locked up in your legs. He's heavy, laying on your hips, and is just waiting for you to pop open so he can under-hook your leg with his massive arm and fling it in a badly executed, but likely very effective, variation of a stack pass. Even if you manage to forestall this happening, your legs are dangling over his hips, and your feet are unable to get a purchase on anything. To make matters worse, your buddy keeps the distance between you shut down to zero. In other words he likes having his knees pressed right up to your butt so that it continues to be difficult for you to move. Have you got the picture? I'm sure you've been there. We all started there! And every time you close your guard you invite this happening.
The key to playing open guard effectively lies entirely in dictating the space between you and your opponent. Lock that in! You must dictate the distance between your butt and his knees that keep creeping forward, and shutting down your ability to use your legs! It's generally not a huge distance, it may be no more than a hand's length, but you need enough space so that you can set your feet on your opponent's hips and brace them off of you with your powerful legs and hips.
The simplest and easiest manner in which to do this is to drop one of your legs while bracing against your opponent with your hands, and shrimp hard (shrimping's not just for escaping mount!) to that side. Let your opposite foot brace like a strut against the opposite hip. Now you have stopped progress and you have a free leg to slide in for your scissor sweep, or knee shield, or one of your other favorite maneuvers.
I'm not going to tell you what to do next, that's your strategy and your game. But I'm going to further suggest that, for many of your favorite moves, this space is going to be necessary. You need a place to drop your opponent when you are looking to sweep him, and you need that space free so that your hips are completely able to swing 180 degrees. Almost all your favorite attacks, chokes, sweeps, and armbars, with damned few exceptions (and then even those exceptions are generally against folks not making themselves difficult), are going to require you being able to make this space and maintain it.
I like to think of it as enough space to allow my opponent to trip. If you're always pressed right up against your opponent they are safe, they can't really fall, they are happy resting on you. With that little bit of space - it may be no more than a curb's distance, you can practice all of the best self-defense jiu-jitsu.
By just opening your guard and dictating that modicum of distance between you, you will open a huge book of opportunity. Give it a try and you will be eventually turn your guard into something dangerous!
Geoff Balme is a second degree black belt and the lead instructor at Open Guard BJJ, Kickboxing, and Self Defense Classes in Apex, NC.