So there's no experimental results to clarify what's going on with quantum mechanics and gravity, but we've also never seen anything travel faster than the speed of light. It has never been observed, ever. Every time we think there's an FTL phenomenon, it turns out to not be FTL. Energy levels are vastly higher in cosmological situations, so it stands to reason that FTL, if possible, would occur in nature. While absence of evidence is not proof of absence, it informs theories. Likewise, proving a negative is difficult or impossible.

The real problem with FTL are the implications it has for the rest of physics. There's a saying: "FTL, relativity, causality, pick two." Built into the logic of relativity is the idea that <c is the maximum velocity for anything with mass, =c for massless particles and >c for negative mass. It's unclear if negative mass is physically meaningful, it has also never been observed.

More than that, built into the logic and math of relativity is the idea that information can only travel at speeds less than c; there are a number of situations in relativity that can arise with superluminal travel that would undermine causality completely; even at near c, time dilation will never alter the order of events, causes will never follow effects and information can never arrive at a listener before the speaker says it.

Relativity is a very well researched and experimentally supported theory. It's only exceeded in predictive power and experimental agreement by quantum mechanics. But neither of them is anywhere near as important to science, logic and reason as cause and effect.

Basically, without proof of FTL that is ironclad, there's every reason to believe that all the rest of science is more accurate, because thre's no explicit experimental proof of FTL and the implications of FTL conflict with all other observations of basically everything.

tl;dr because of the implication