I am in no way saying that this is cheap but 300 TB will set you back a little less than $6k with tax. Very attainable for people other than OpenAI and Facebook. And it's not crazy at all to snag a server with enough bays to house all those.
The cost of rest of the hardware, running it constantly, and 'admin' overheads aren't to be scoffed at to be fair.
I have a Supermicro 24 bay 2U in my house with an array around half that size in it. It’s not prohibitive.
For reference, considering you can purchase a 12-month Spotify Premium subscription via a $99 gift card at the moment, that same $6k could be used for 60 years of Spotify Premium.