Why is Quantum Field Theory considered more fundamental than General Relativity?
It isn’t. But QFT and GR are very difficult to combine, and an assumption is commonly made about which one will have to give. The assumption typically made is that GR will need corrections at very small scales or very high energies, largely because it assumes things are smooth at all levels (it is classical) whereas QF assumes things are lumpy. The principle is that things cannot be simultaneously smooth and lumpy. I do not think this means GR is seen less fundamental, but it is an indication that it is seen as most likely to need modification. Remember, QFT is the most accurate theory ever created, and the standard model uses it with remarkable effect.
