Striking is inherently more difficult if you look at what makes you a good striker. Much of what a makes a good striker a good striker are natural gifts such as quickness, durability, coordination, and reflexes. It isn't that you can't improve those things, but just that it involves a lot more attributes than grappling does. Plus, getting hit in the face and still being able to strike is harder than grappling, thus why 90% of the time when someone is rocked, they immediately shoot for a takedown or clinch up. Defensive grappling and stalling is easy to learn and can be drilled constantly. Sparring too much can be a detriment to your health in a big way.
My grappling experience introduced me to a ton of guys who have crap vision, poor reflexes, and aren't what you would consider "tough", but they get into that flow state on the ground and force you to operate at their pace, plus, they don't have to worry about getting tagged in the face or GNP, so they are calm and care free.