It depends on the person. Also, the phrase "harder to learn" is too broad, learn to what level? And under what circumstances? What form of grappling? What form of striking? I'm not nit picking, these questions matter, these specifics actually change the answer to the initial question.
Defensive grappling is easier to learn than defensive striking, & that's generally speaking. In saying that, if you want to learn to stop the NCAA DIV 1 champ from taking you down, that defence is going to take a long long long time to learn. Then, how long will it take you to beat that NCAA DIV 1 champion in a straight wrestling match? Probably forever, so it can't get much harder, by definition, than that.
Striking in terms of boxing, it is very hard to learn how to defend at a high level, but that is within the realm of pure boxing. Meaning: If you are forced to BOX with prime Tyson in a ring without holding onto him, it is going to take you you're whole life to learn how to survive, and then you probably still won't. Now, if you are allowed to tie him up, then it makes it easier obviously. Some striking arts, involve a tie up as an offence, and as a defensive technique. So it matters which specific art/sport we're discussing.
The argument of strikers moving to grappling easier than grapplers moving to striking isn't valid in my opinion. The nature of being fundamentally sound striker does not go against the fundamentals of grappling, also as prior mentioned, generally speaking grappling DEFENCE is easier to pick up than striking DEFENCE, which means if the striker survives the better grapplers submissions, which is often the case, that is why. Now, on the other hand, we don't often see "strikers" submitting "grapplers", but we do see "grapplers" knocking people out after learning how to strike.
The phrase is too broad, it is a complicated question with no universal answer in my opinion. Way too many variables to consider.