I feel like you could ask similar "why" questions about everything design-related. "Why is no one asking why good-looking, modern designs are necessary?"

Many people interpret quality of design as an indicator of content or product quality.

To address your particular question, depth creates a more realistic and visually interesting representation of information on a screen. It can also serve to indicate the priority of information on a page. If depth draws your eye to or away from a particular element, it can help to achieve the designer's goals.

This is how you get more "trendy" designs than something timeless when fundamentals are thought through.

Depth is a fundamental.

I can’t speak to this suggestion but I purchased the book and watched the videos and all of the guidance and material I remember is very sound and, in my experience, very enlightening for me as an engineer who’s worked in SaaS with very talented UX people but still struggle with coming up with clean UX on my own.

Over many years, I've come to know that "Design" is a very subjective field. There are efforts to make meaningful measurements, A/B testing, etc but the domain space of possibilities is so large that it is impossible to test all configurations - layout alone has endless possibilities and which layout is better? My guess is as good as the designer's. I need to be able to see either one of the following to consume the advice - 1) Deep logic that describes the fundamental problem, an unchained reasoning leading to the solution with little ambiguity or "gray areas". or 2) Concrete experimental or empirical data that a particular solution works while making sure things like accessibility are accounted for. Otherwise, how do I know that some authority in "Design" is correct and I should follow their advice?

I am just speaking my mind, what kind of things prevent me from trusting others despite of their benevolent intentions. Time and again, I've been bitten by bad subjective advice - there is a lot of bullshit out there. Tread carefully, adopt what makes sense to you and ignore others unless evidence shows otherwise. That said, there are a lot of gems out there as well.

If the thing I am making is art and is subjective, then why can't I just be original? If a particular activity is totally arbitrary, then all bets are off.

Blindly following advice is why we as a society get stuck in a local optimum and don't try other things. Domain exploration and experimention, originality and authenticity are paramount to getting out of the local optima. Every once in a while someone does and they revolutionize the world.

Just because you don’t know what depth means doesn’t mean it’s wrong.

Anyone who can draw knows what that means.