Bas Rutten talked about this one of the times he was on Joe's podcast. Lots of young guys ask him whether strikes to the thigh or arm are effective and why people don't do them more often in positions like that one. He said "wait till you get in the ring and you'll see".
And apparently, according to him, it's the jitters. It's like stage fright. When you get into the ring, all creativity and peace of mind and rational sense go out the door and you're stuck doing whatever you drilled over and over in the gym till it became instinct. A few guys like Jones and apparently Tim Means can overcome that and get creative in there. Maybe drugs have something to do with it.