Youtube had a space devoid of competition. The next guy doesn't. If the next guy wants to compete, they have to have all the features of Youtube or people will complain. Many of Youtube's current features cost money and weren't present when Youtube started.

The space is also more regulated now that Youtube exists, meaning the new guy has to follow regulations which normally costs money. When Youtube started, those regulations didn't exist, because Youtube didn't exist.

Youtube got big by building a city in an open field surrounded by nothing but open fields. The next guy has to build a city directly next to Youtube, follow all the same laws as Youtube, and ask you not to drive into Youtube.

Two reasons:

1. Because no one else occupied the same space in a meaningful way.
2. Low interest rates meant they were able to get massive investments without the burden of profitability.

Now you'd need to distinguish yourself from YouTube in a meaningful way as well as provide a sustainable revenue model, such as advertising, in order to gain access to a similar amount of venture capital.

Did youtube at the time serve millions of users daily and stored a gargantuan amount of petabytes worth of videos?

Even if a competitor rises, they will need money somehow, and in this hell of a capitalist world, only big corporations have it.

They were big through investors throwing money at a money sink for years. Youtube was losing tens to hundreds of millions of dollars a year for a long time, before it finally became profitable.

A new competitor wouldn't get such favorable support from investors.