If you take some time to improve your reading comprehension skills, you will notice I didn't say direct experience alone is valid knowledge. It must also be able to manifest in reality.
Unlike the man in the insane asylum I mentioned earlier in the thread who's direct experience tells him that he is Napoleon. He has this direct experience, as do the Christians concerning their creator god, but it needs to be validated. How come you only need your direct experience, but they require more?
So to claim knowledge, on one hand you need direct experience of what you are claiming, and on the other you need to demonstrate that this aligns with reality.
As Adamantine seems to have bowed out, let me give you a plausible answer.
You can classify phenomena in several categories which go till very hidden phenomena. Still, even this last kind can be verified by experts.
If we were talking about physics, for instance, if someone says there are planets orbiting stars beyond our solar system, it won't be the average Joe that can refute or confirm this. You'll need and expert who can collect and interpret data. Agree?
So, if we talk about subtle phenomena like what happens after death, we are talking about something that not everyone can verify at this moment. Let's assume Adamantine described one of his experiences. To refute it or confirm it, we would need an expert meditator.
So these subtle phenomena manifest in reality, still not at a level that we all can easily see. We need training to confirm or refute them, that's all.
Now let's assume Adamantine says that hares have horns. You won't need an expert to say that there's no such thing. Of course there's the possibility of somewhere in a hidden forest there are hares with horns, although quite unlikely as there doesn't seem to be any evolutive advantage about it neither their ancestors had them... nevertheless in some special environment due to a series of mutations such characteristic could be selected. Still, it's something we could all see easily. It's not a very hidden phenomena. I don't need a doctoral degree to confirm or refute this, neither do I need to be an expert meditator. I just need to see one and know it's not a fake.
Now, if I claim to be Napoleon, that's not a very hidden phenomena. Napoloeon or Jesus or whomever you decide to choose are dead. If I say I'm one of them, it's obvious that I'm not. No training is needed to confirm or refute it. Answering directly to your question, this is why "they" require more. One doesn't need to be an expert to dismiss these claims.
So the difference lies in the class of phenomena we're talking about. To verify some one needs years of training while others anyone can verify without specific training.
So on one hand Adamantine would have direct experience and on the other the phenomenon indeed aligns with reality. The fact is that there are aspects of reality that aren't easily accessible to everyone.
Knowing this difference would be enough for you to answer your own question. You asked a question that had no need to be asked because your premise assumed all phenomena having equal degrees of subtlety while, in fact, they don't. This goes for Buddhism or Science. In the end it's a matter of verification. Even very hidden phenomena may be verified when one was the necessary qualifications. This also goes both for Buddhism and Science.
The problem is that people who tend to prefer a "lite" version of Buddhism also seem to show some preference for a "lite" version of science (a kind of pop science pseudo skeptics seem to love), which utterly disregards philosophy of science. Confusion is expected when both meet.