For a very large set of people, motivation to study is absolutely key. If your motivation comes directly from learning the concept itself, you may be right that studying it directly is more effective. But for many, trying to do that because it's "better" will lead to worse outcomes when they end up not completing the study.

Calling it "an excuse to slack off" ignores that different people's motivations are different. If being interested in poker makes someone want to learn the underlying math, then that probably is the most effective way _for them_ to study it.