No one's stopping claimants, or anyone else for that matter, from being tested under proper observing conditions (ie, where all opportunity to cheat is eliminated), yet none of the claimants attempting the MDC have done so.
This is, not to put too fine a point on it, a stupid comment.
You're asking that claimants attempting the MDC should have passed the MDC already. Part of the issue is that "proper observing conditions (ie. where all opportunity to cheat is eliminated)" is one of the specialist skills that Randi and company provide. One of the things he has said (repeatedly) is that scientists should consult magicians when studying the paranormal. Well, Randi is a highly skilled magician and available more or less for free --- why should I pay money for a prior consultation with a less skilled magician who doesn't specialize in detecting cheating?
Setting up "proper observing conditions" is hard work. Why should the claimants have to do all that hard work by themselves, without benefit of Randi's knowledge and expertise, in order to avail themselves of it later?
Oh, and a possible paranormal ability that can't be easily tested within a day? "I can make plants grow faster/taller/bushier." (Which is a genuine claim that people have made.) At a minimum, this will require several weeks of work and observation; if the plants in question are redwood trees, it could take several hundred years. Pyramid power --- "if you store razor blades in this, they will stay sharp for longer" --- is another classic woo claim that would be be hard to test; an unused razor blade will stay sharp for months or years, depending upon the climate. Sure, it would be paranormal if one stayed sharp for two years while the other only stayed sharp for six months ---- but you're not going to notice a difference after a day or a week.
If the JREF uses a day as their threshold, even simple claims like "drinking this magic water will make you resistant to disease" are untestable.