Question: Most Yeshivos (from 5th Grade-End of High school) Learn Talmud Bavli.

My question is: Why? What is the point of learning Talmud Bavli so extensively? 

I know that it is Hashem's Word, but there are a lot of other things to learn which are more useful (ex. Shulchan Aruch Harav where he gives the reasons (mostly from the talmud) and gives the halacha) and are Hashem's words? 

So why specifically learn Talmud Bavli? And why spend so much time on it? 

Indeed, there is a reason that we would not learn Talmud Bavli; that we know that the whole point of learning is actual performance of mitzvot, so Talmud seems ineffective vis-à-vis e.g. Shulchan Aruch Harav

Answer: 
Yeshivos start learning Gemara too early (the Maharal said this already many centuries ago). However, the overall goal of the Yeshiva curriculum is to train the student to be able to learn Torah She Baal Peh on their own in their lifetimes - specifically the 613 Mitvos and their details.

Before Torah She Baal Peh was written down, the Yeshiva had to teach the student the whole thing and get them to memorize it. Now that it is written down the goal has changed to enabling the student to read and understand it, continuing the study of it after they graduate.

The main key to that level of learning is the Talmud Bavli. Without being able to learn that, there is no way to be able to learn more.

Of course curriculum will vary in how they get there and additional goals they want to teach students. But the Talmud Bavli part is about that "learning how to learn" process. Learning Halachos alone will not enable that.

(One Source which discusses this goal of Yeshiva learning is Hilchos Talmud Torah by the Alter Rebbe of Lubavitch).

The above should not be taken as an endorsement of a specific curriculum, or a specific amount of time spent. There is no one right answer in how to achieve that goal, and קנאת סופרים תרבה חכמה - competition in schooling increases scholarship.
Talmudic style uses the Socratic method of learning, which is somewhat unique. First, it encourages students to learn from each other rather than from the teacher ("chevruta" style). Second, the style of learning in the Talmud is based on questions and answers - proofs and rebuttals - much like a debate. This style is unique to the Talmud. The Torah doesn't have debate, and Shulchan Aruch and Ramba"m, as examples, don't use this style either.

Gaining these two skills - cooperative learning as well as debative / "argumentative" logic is a critical skill in terms of learning. The first skill helps develop friendships and understanding how to deal with peers as well as a diverse audience. While I don't think the yeshiva's goal is directly to promote skills that will be useful in business, this skill does end up achieving this.

The discussion / argumentative skill is a "mixed" end-result. Understanding how to question and disect a problem is an important skill as it teaches you not to always take things at face-value but to research it. On the flip-side, I've also seen this skill be detrimental when teenagers use this style to debate parents' and elders' requests or when they over-analyze an issue that isn't or shouldn't be debated.