The statement that India and the USA have become such strong strategic partners that they need not become formal allies accurately reflects the evolving dynamics of their relationship. While their partnership has deepened significantly across various domains, both nations, particularly India, have compelling reasons to prefer a strategic partnership over a formal, treaty-bound alliance.

Strengths of the Strategic Partnership:

1. Convergence of Interests: Both countries share a growing convergence of strategic interests, particularly in maintaining a free, open, and inclusive Indo-Pacific region, countering China's assertive rise, promoting maritime security, and combating terrorism. This alignment of interests forms the bedrock of their cooperation.
2. Deepening Defense Ties: The partnership has seen unprecedented growth in defense cooperation, including joint military exercises (e.g., Malabar), intelligence sharing, and defense technology transfers. India has signed foundational agreements like LEMOA (logistics), COMCASA (communications), and BECA (geospatial intelligence), which facilitate interoperability and deepen military-to-military engagement.
3. Economic and People-to-People Links: Robust trade, investment, and a large Indian diaspora in the US further strengthen the relationship, creating multiple layers of engagement beyond just security.
4. Multilateral Cooperation: Both nations collaborate closely in various multilateral forums, including the Quad (Quadrilateral Security Dialogue), G20, and the UN, to address global challenges.

Reasons Against a Formal Alliance:

1. India's Strategic Autonomy: A cornerstone of India's foreign policy since independence has been strategic autonomy and non-alignment. A formal alliance, with its binding mutual defense clauses (like NATO's Article 5), would severely constrain India's foreign policy options, potentially entangling it in conflicts not directly related to its core national interests. India values its ability to maintain independent relationships with countries like Russia and Iran, which might be complicated by a formal alliance with the US.
2. Historical Context: India's historical experience and its commitment to non-alignment are deeply ingrained in its diplomatic ethos. Shifting to a formal alliance would represent a radical departure from this long-standing tradition.
3. Flexibility and Issue-Based Cooperation: The current strategic partnership allows for flexibility. Both countries can cooperate on issues where their interests align without being bound by rigid commitments where they diverge. This 'partnership of choice' enables a more agile response to evolving geopolitical realities.
4. Differing Threat Perceptions: While both perceive China as a challenge, their primary security concerns and geographical contexts differ. India's immediate security environment includes Pakistan, and its strategic calculations are distinct from those of the US.
5. US Perspective: From the US perspective, a formal alliance might not be necessary to achieve its objectives with India. The current level of cooperation, without the formal obligations, might be seen as sufficiently effective, allowing India to maintain its unique position in the global order.

In conclusion, the current strategic partnership between India and the USA is a pragmatic and effective arrangement that leverages their converging interests while respecting India's deeply held commitment to strategic autonomy. It allows for robust cooperation across critical sectors without the constraints and obligations of a formal alliance. This 'quasi-alliance' or 'partnership without alliance' model serves both nations well, providing the necessary flexibility to navigate a complex and multipolar world.