Some people (including me) care about software freedom. The ability to fork and redistribute software while continuing to publish any changes to the code is great.
Not using an open source license but a source available license is not something that I like to see, but it’s their right to do so. There’re enough open source YouTube frontends like NewPipe and LibreTube.
PS: What I really don’t like is them using the term open source. Open source is a well known term that’s well defined. Source available describes exactly what this app is without implying the freedoms associated with open source.