Most players who play the crosscourt corners do exactly that - they play the crosscourt corners. And that's where many or most of your attacks should go, as they give the most table when hitting crosscourt. However, when looping (where the topspin pulls the ball down so you need less table), against shorter balls (where you have more angle), or when attacking from a wide corner (also giving more angle), you should be doing more than just going to a wide corner - whenever possible, you should go outside the corner. This can add an extra foot for the opponent to cover - and allows you to run him ragged.
The key here is that you can't learn to do this unless you actually spend time (drum roll please) practicing it. That means challenging your instincts and going for wider and wider angles on your shots, pushing the limit, and often missing as you gradually gain an instinct for just how wide you can safely go. You also have to gain an instinct for how to follow it up against various opponents. Once you go to a wide angle, the seemingly logical thing to do is go the opposite way on the next shot, and that usually works. But many players expect that, and after going wide, immediately move to cover the other wide angle. And so it's often best to go wide to the same spot twice in a row. Experiment and you'll gain an instinct for this.
It's not just in rallies. If an opponent serves short, why not take it off the bounce and make a wide-angled return that goes outside the corner?
During my peak years, one of my primary weaknesses was anyone who could attack my wide backhand outside the corner. I was a wall up to the corner, from many years of training. But since I rarely drilled against balls outside the corner, I tended to lunge for them even though I had time to move into position. And like me, many players are so used to covering the area from corner to corner, so when you make them go outside that, they often fall apart. And that's what you want!