First of all, no one is demeaning the kedusha of Eretz Yisrael. I just feel these questions make as much sense as asking someone eating an apple; why don't you eat an orange? This is not the place to discuss how great these tzadikim were; everyone knows, whether or not they'll admit to it. If someones relative or Rabbi is buried outside of Israel and in a land soaked with Jewish blood, does that mean the grave should never be visited? That would be cruel, for a departed loved one to have no one come pray at their grave. The benefits for the soul of the departed, when people pray at the grave, are great.
When someone travels to Europe on business or goes to Bermuda on vacation to we ask: Why not Israel? How many people would turn down an opportunity to make money dealing with a company based in one of these countries? (basically the whole Europe) Besides, who says that these people aren't going to Israel for Lag Baomer? Many do. Actually, I think the people with the questions are probably ignorant about the idea of praying at the graves of tzadikim. They do not realize that many people go there praying for help from Hashem and asking the tzadik to daven for them or loved ones for yeshuos in health, parnasa, shiduchim and more.
One more thing, in Likutei Mohoran-2 109, it says that the place where true
tzadikim are buried have the kedusha of "Eretz Yisrael Mamash". So, they
really are going to Israel.
If anyone has more to add, please comment.