Full Idea
Gravity is not included in the Standard Model because we simply cannot study it using particles.
Gist of Idea
The Standard Model omits gravity, because there are no particles involved
Source
Gavin Hesketh (The Particle Zoo [2016], 09)
Book Reference
Hesketh,Gavin: 'The Particle Zoo' [Quercus 2016], p.233
A Reaction
I'm guessing that Einstein describes how gravity behaves, but not what it is.
Related Idea
Idea 21177 String theory offers a quantum theory of gravity, by describing the graviton [New Sci.]