Einstein's geometry is often explained to the layman (me) as "Mass tells spacetime how to curve, spacetime tells mass how to move". It seems to me then, that what we call gravity is explained by the movement of a body in a straight line through curved spacetime.
I understand from similar questions that it's a model, we use the apparent force of gravity because it's easier to model a force than model curved space,
The standard model of particle physics explains the other 3 fundamental forces and the particles that carry them. However it can not explain gravity as a force in the same way, and the predicted graviton hasn't been detected by any of the particle accelerators.
In light of the fact that it is explained by the curvature of space, that it isn't explained by the otherwise well tested model of particle physics, that it is completely different from the other 3 fundamental forces in magnitude, why is it still considered to be fundamental?