The big problem with Google is that they are in, or a part of, almost everything on the internet, and it all funnels users back to them one way or another.
Their search favors their own things, so if you search for anything, YouTube will come up most of the time. This by itself is enough to kill competition. Their search also recommends their browser heavily if you're not using it, which is how they became the most-used browser, which defaults to their search, which by default recommends YouTube in most searches.
Even if you don't use Chrome, don't worry because they will pay absolutely nuts money to be the default search on their competitors browsers, which is again more people to YouTube. And if that isn't enough, most browsers are built on Chromium, which Google maintains, meaning they can sway the course of their competitors browsers over the long term, which they are doing by selectively killing and bringing in certain technologies over years.
Android, which is also Google, I believe has YouTube installed by default, or at least all of my phones have had it. Trying to compete with defaults is almost unachievable. It's easy to think that people will change settings, but most people don't.
I agree that the technology and infrastructure needed to run YouTube is huge, and it's amazing, but that's only part of the story. Google has so much control of so many things that even if you could build the same thing, that's only the beginning.
But it's not only YouTube, it's the same for Gmail. Gmail has so much market share that they can kill competitors by making another email service seem unreliable. And all of their services point back to Gmail.
It's not just that they have a monopoly on video, they have a monopoly on the whole internet.
