1. Because we (the Agudas Yisroel, etc.) oppose the very existence of the State. We cooperate with it since it de facto exists. And we don’t advocate its dismantelment since once it was born destroying it completely is worse. But we minimize our interaction and cooperation with it to the bare minimum.
2. Because anyone who wants to learn Torah full-time is entitled to do so even if he is a poor learner. All that is to be expected of him is to DESIRE to learn Torah.
3. Because the State Army is full of pritzus and will inevitably make many frum draftees into chilonim.
4. Because Torah learning is the true and best protection for Klal Yisroel, much more effectively than being a soldier.
5. Because the Army doesn’t need us. They are doing fine without us. Even their own Generals publicly admit that drafting Chareidim will NOT help the Army and, in fact, will make the Army’s job more difficult.