I'm not sure what you mean by saying that quantum gravity "doesn't exist". But because this is the Astronomy SE, I will interpret your question as primarily asking why astronomy hasn't found evidence of quantum gravity. This is a reasonable question; after all, nineteenth-century astronomers have found evidence of funny business in the perihelion precession of Mercury, which in the twentieth century was understood as the relativistic corrections to the Newtonian orbits. Then astronomy one-upped that and found more evidence for general relativity.
So why not now? Because gravity is weak. Very weak. Weakness per se isn't a big problem theoretically; it makes it possible to calculate quantum gravity corrections when they aren't too large, which can be interpreted as a running Newton's constant:
$$G(r) = Gleft[1 - frac{167}{30pi}frac{Ghbar}{r^2c^2} + ldotsright]text{.}$$
For example, for the Sun's gravitational field near the Sun's surface (mass $M_odotsim 2times 10^{30},mathrm{kg}$, radius $R_odotsim 7times 10^8,mathrm{m}$), the relativistic corrections are of order $GM_odot/rc^2sim 10^{-6}$, a few parts per million, while the quantum gravity corrections are on the order of $Ghbar/r^2c^3sim 10^{-88}$, which is so much smaller as to be completely hopeless.
Making the Sun a black hole would not improve things by much, since in that case $r = 2GM_odot/c^2$ means while the relativistic corrections become appreciable, the quantum gravity corrections are on the order of $10^{-76}$. Note that larger black holes are actually worse, which is sensible because large-scale quantum gravity must agree with general relativity.
An interesting overview by Cliff P. Burgess can be found Living Rev. Relativity 7 (2004) 5, here.
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