The problem with considering quantum mechanics a theory of everything lies in the fact that it hasn't yet been successfully integrated with gravity. Quantum mechanics, as it is currently understood, provides a framework for describing the behavior of three fundamental forces: electromagnetism, the strong nuclear force, and the weak nuclear force. However, it hasn't been able to incorporate gravity into its formalism.
In quantum mechanics, forces are described by the exchange of particles called force carriers or gauge bosons. For example, photons are the force carriers of electromagnetism. These force carriers mediate the interactions between particles, and their behavior is successfully described within the framework of quantum field theory.
Gravity, on the other hand, is described by general relativity, which is a classical theory of gravity formulated by Albert Einstein. In general relativity, gravity is understood as the curvature of spacetime caused by mass and energy. Unlike the other forces, which are described by quantum field theories, gravity hasn't yet been successfully quantized. Attempts to naively combine quantum mechanics with general relativity encounter mathematical inconsistencies and difficulties.
The challenge in reconciling quantum mechanics with gravity arises due to the different nature of the two theories. Quantum mechanics deals with discrete particles and probabilistic behavior, while general relativity describes the smooth curvature of spacetime. To achieve a theory of everything, physicists seek a framework that can consistently describe both the microscopic quantum world and the macroscopic curved spacetime of gravity.
Various approaches, such as string theory and loop quantum gravity, have been proposed to address this issue. String theory, for instance, attempts to unify all the fundamental forces, including gravity, within a single theoretical framework. However, it is still an area of active research, and its ultimate validity and ability to provide a complete theory of everything are still subjects of investigation.
So, while it is true that gravity can be described as a geometric property of spacetime in general relativity, the challenge lies in finding a consistent and mathematically rigorous way to incorporate gravity into the framework of quantum mechanics. Until such a theory is developed, we cannot consider quantum mechanics alone as a complete theory of everything.