I think there are a few benefits to doing creating a whole new app. First, a video editing application can become very sophisticated, especially if you want to both offer easy fast creation/editing features but also provide a spectrum where users can opt into more customization or sophistication. Cramming that user interface alongside other UI for slides or whatever will quickly become too crowded and unmanageable, for users but also the teams trying to add more functionality. The second benefit is that a new product lets you have independent flexibility on pricing or usage models. Third, you have the opportunity to perform impactful marketing around something new, as we’re seeing here. Finally, you get to define how users should use the product and incorporate it into their world. If these same features were just in Slides, people may automatically view video creation as a tool to generate content for live slide-driven presentations. But as a separate app, you have a chance to train users to view this as a daily tool that they use for all sorts of communication - both casual everyday chat-like messages and highly-polished presentations.
I have no idea which of these benefits Google is intentional about, by the way - I am just speculating.