Because it ties in two barriers to entry that have rarely existed together before: minimum efficiency scale and network effects.
Facebook only has network effects, there isn't anything particularly efficient or advanced about what it does. Given its minuscule infrastructure when compared to Google and Amazon, it would be very hard for it to achieve the same level of cost that Google has on YT. Pure search doesn't rely so much on scale as well.
Any other provider doesn't have either the network effects that YT does nor the scale to be able to compete with Google on YT.
From a scale perspective, the only companies who could effectively compete with Google would be Microsoft and Amazon. Microsoft is still trying to figure itself out, especially on the consumer side. Amazon is geographically limited and has very little expertise in open content and ad-funded platforms, but it is trying with Twitch.
Facebook only has network effects, there isn't anything particularly efficient or advanced about what it does. Given its minuscule infrastructure when compared to Google and Amazon, it would be very hard for it to achieve the same level of cost that Google has on YT. Pure search doesn't rely so much on scale as well.
Any other provider doesn't have either the network effects that YT does nor the scale to be able to compete with Google on YT.
From a scale perspective, the only companies who could effectively compete with Google would be Microsoft and Amazon. Microsoft is still trying to figure itself out, especially on the consumer side. Amazon is geographically limited and has very little expertise in open content and ad-funded platforms, but it is trying with Twitch.