Because we exist. Quantum field theory (QFT) does a great job describing the electromagnetic, strong and weak nuclear force, and has done a spectacular job des

Because we exist. Quantum field theory (QFT) does a great job describing the electromagnetic, strong and weak nuclear force, and has done a spectacular job describing reality in ways too numerous to count. But it has two principal failings. The first is that every time someone tries to incorporate a quantum field theory of gravity, the inevitable conclusion is "the first time a graviton interacts with a particle, the entire thing collapses into a black hole". Because we haven't collapsed into a black hole, we know our theory is incomplete. The second is the vacuum catastrophe. QFT predicts a value of the zero point vacuum energy (very closely related to dark energy, aka the cosmological constant) of roughly 10^112 ergs per cubic centimeter. With a bit of a hack, we can get this to cancel out to precisely 0. However, the observed value of the zero point vacuum energy is roughly 10^-8 ergs per cubic centimeter. Which isn't even close to 10^112, but more importantly, it is also not zero, so our hack to cancel it out doesn't work.