59 At length the time arrived for obtaining the plates, the Urim and Thummim, and the breastplate. On the twenty-second day of September, one thousand eight hundred and twenty-seven, having gone as usual at the end of another year to the place where they were deposited, the same heavenly messenger delivered them up to me with this charge: that I should be responsible for them; that if I should let them go carelessly, or through any neglect of mine, I should be cut off; but that if I would use all my endeavors to preserve them, until he, the messenger, should call for them, they should be protected. (Joseph Smith History 1).
And yet, we see that Helaman was given the same covenant nearly two thousand years previous regarding the plates by his father:
14 And now remember, my son, that God has entrusted you with these things, which are sacred, which he has kept sacred, and also which he will keep and preserve for a wise purpose in him, that he may show forth his power unto future generations.
15 And now behold, I tell you by the spirit of prophecy, that if ye transgress the commandments of God, behold, these things which are sacred shall be taken away from you by the power of God, and ye shall be delivered up unto Satan, that he may sift you as chaff before the wind.
16 But if ye keep the commandments of God, and do with these things which are sacred according to that which the Lord doth command you, (for you must appeal unto the Lord for all things whatsoever ye must do with them) behold, no power of earth or hell can take them from you, for God is powerful to the fulfilling of all his words.(Alma 37).
This covenant has been upon these plates at least since the times of Nephi, although I wonder if they were on them before that and that has to do with Laban's untimely demise because he was dealing with them wickedly - they were in his personal treasury before they were obtained by Nephi.
As Joseph Smith noted as to why he received this charge:
60 I soon found out the reason why I had received such strict charges to keep them safe, and why it was that the messenger had said that when I had done what was required at my hand, he would call for them. For no sooner was it known that I had them, than the most strenuous exertions were used to get them from me. Every stratagem that could be invented was resorted to for that purpose. The persecution became more bitter and severe than before, and multitudes were on the alert continually to get them from me if possible. But by the wisdom of God, they remained safe in my hands, until I had accomplished by them what was required at my hand. When, according to arrangements, the messenger called for them, I delivered them up to him; and he has them in his charge until this day, being the second day of May, one thousand eight hundred and thirty-eight. (Joseph Smith History 1)
I always wondered why we don't just put these in a museum somewhere to convince the world....but seeings what happened with Joseph Smith having the plates, and how long this covenant has been in place regarding the plates, and the purposes for them - I think we can see why we don't. People would be breaking in to steal them, all sorts of chaos, and there are things in them not yet meant for the world - it would come down to them only being able to be protected by the power of God himself (and then lots of lawsuits against the church with people claiming we were shocking people) and the Lord's work just isn't meant to be sensationalized like that.