The big ones do have an intro, whether it be by an actual video intro or more in the likes of them just saying an intro. An example on PewDiePie: "Hey how's it going bros, my name is PewDiePie!" or maybe Boogie with his vlogging series: "What's up, ladies and gentlemen on Youtube. Boogie2988 coming at you live once again once again through the power of the internet". This right here is where the content creator is using their words to introduce themselves, where as others would use an intro made with images, their channel name and then slapped on with some music to help draw the viewer in.
So if you look at why the big ones you mentioned doesn't use an intro where it's like a video, it's because the way of introducing yourself via words over and over and over again, like a salesman. You will slowly remember them more and what the intro for them will be, because they use only their words in a very short amount of time. Where as the video is quicker to draw you in, but has a less way of making you remember it.
I myself do it with words to introduce myself and my slogan in the videos I create. However, to sum it all up....they do have an intro, it's just not being shown in the way you think.
Hope this helped ya out a little mate, stay cozy and have a nice day!