There aren't many barriers to entry in this field though. It's not a situation like a utility/ISP where you can only feasibly have one last-mile connection due to permits or limited radio spectrum. It's not even something like a nuclear reactor which costs $10B and a decade before you can flip the switch.

I'm not saying I could build a streaming site in my closet this weekend, but it's not impossible for competitors to enter the space (or existing competitors to grow in the space) so long as Google/YT doesn't too anything too anti-competitive.

I have to disagree with this strongly. There are significant economies to scale in any streaming service. For example, in terms of costs (streaming services require a lot of hardware). But also in terms of branding and reaching a critical mass in terms of content.