That is incorrect. Not only can things be demonstrated over the internet but things routinely ARE demonstrated over the internet. Want to fix a leaky tap but don't know where to start? You can find it demonstrated on the internet? Want to make a Proton Pack for your Ghostbuster costume? You can find it demonstrated on the internet.
Advances have been made of our understanding of zoology and cryptozoology via the internet as well - consider the significant roles played via the internet of the ivory-billed woodpecker and "Bigfoot Skookum cast" episodes. Despite being initially seen as strong evidence of extinct and undiscovered species respectively both were demonstrated to actually be evidence of other more mundane creatures. Even the top notch highly qualified people who let their beliefs guide the evidence (rather than the other way around) got burned on those which provides an important reminder that all people without exception are prone to get carried away with what might be (the possibilities) rather than what is (the probabilities). It is human nature and we see it time and time again yet it is something that we need to consider when evaluating evidence.
If Bigfoot exists then it too can be demonstrated on the internet and that is exactly what people attempt to do - there is a proliferation of what is claimed to be Bigfoot evidence available on the internet. Is it evidence of a giant undiscovered species? No - but it IS evidence of a false belief. There is an obvious disconnection between what people are claiming/experiencing with what is presented as supporting evidence. It is precisely this disconnection between subjective experience and objective reality that is poorly understood yet it lies at the heart of the mystery (most mysteries, actually). Is the Truth "out there" (in some paranormal/supernatural realm) or is the Truth really "in here" (in our own heads)?
It is fun to speculate - to utilise on'e creativity in order to explore the many fantastic possibilities and that alone has a tremendous appeal to people (here we all are in our amazing diversity discussing these very things). It IS adventurous but it also that appeal that leads often leads us astray from how things actually are (as in the examples above - and there are many, many more examples from almost every field of human endeavour). By rigourously examining the claims and the actual evidence and even the frequent disconnection between claims and evidence we CAN contribute to the broader knowledge on the subject. The wide variety of people here on this Forum and elsewhere provides a broad cross-section of folk-expertise and it one of the great pleasures of the internet experience is finding people who do have a great deal of information/knowledge on a whole range of obscure subjects and are willing to SHARE that information/knowledge.
Let us not forget that internet communication is only a relatively new thing - one that allows us regular slobs from all over the world to engage and interact in almost any and everything for the first time in human history and this very same discussion about Enlightenment v Enchantment has been going on for at least the last several hundred years. In effect, we are all new players in this ongoing social drama and the action is frequently rough and ready but that is a reflection of who we are as a loose community.
Folklore studies talk of the friend-of-a-friend (FOAF) network that originates and perpetuates urban myths - the original source is almost never known and the stories seem real because they get passed along by close source (a friend or a friend-of-a-friend) of the supposed original source. The internet often brings us closer to the "original source" of incredible stories/mysteries as they happen. We are at the coal-face of unexplained mysteries here. We CAN make a difference by our involvement on the internet by sharing what we learn "out in the field" or "from behind a computer screen" via demonstration and providing accurate information.
Of course, people can believe and say what they want but, quite frankly, there are also a lot of wacky and false claims/people out there so it is more about what the objective evidence can tell us about what is actually going on. When beliefs and evidence are impossible to separate or are inexorably linked then it is important to examine that as well...