They also acquired Android, Docs, Sheets, Slides, and many of the precursors of Maps & Ads. There's value in scale. The YouTube founders themselves said they

They also acquired Android, Docs, Sheets, Slides, and many of the precursors of Maps & Ads. There's value in scale. The YouTube founders themselves said they couldn't have grown YouTube without help from Google infrastructure and other business functions. In the mid-00s people were just figuring out how to run an Internet business with billions of daily users; this is a service now externalized through Cloud providers (including Google), but back then "not falling over when you got Slashdotted" was a key competency that Google had but many of the thousands of other Internet startups did not.