The value of YouTube isn't purely monetary. Controlling YouTube is extremely advantageous for Google, even if it isn't particularly profitable.
Besides the general advantage of having control over such a massive platform, which definitely plays an important part in the lives of hundreds of millions of people, Google likely views YouTube as important to control. If YouTube were a separate entity, it could e.g. freely choose their ad providers or even provides ads themselves, essentially creating competition for Google. Google also has trivial access to the data there and therefore the easiest access if they want to train AI on that data. Last but not least I think Google sees YouTube as vital for their corporate image and their social mission presented in Google Jigsaw.