According to this website (which I know nothing of its credibility--take it for what its worth--so perhaps someone else can corroborate whether it is accurate or not) this is not entirely true. Yes, there was a leasing of the land in 1934, but unlike what you said Cuba has indeed made many attempts to revoke the agreement. The article mentions (at least) one legal crux:

The issue of returning Guantánamo to Cuba is complicated by the agreement signed by Batista in 1934. The agreement states: “Until the two Contracting Parties agree to the modification or abrogation of the stipulations of the agreement in regard to the lease to the United States of America of lands in Cuba for coaling and naval stations… the stipulations of that Agreement with regard to the naval station of Guantánamo shall continue in effect.”

So, Cuba has tried to get the land back but the agreement states that both parties must agree to terminating the agreement, which the US refuses to come to the table to do.

I think that there is still a good argument to be made why the US should give up gitmo, despite any lease agreement, but I don't want to argue that now. I just wanted to mention that your description of the state of affairs may possibly not be accurate. Hopefully this is helpful for the discussion here.