Noise spikes induced into the unshielded wires of UTP network cables are detected and eliminated by noise filters in the end devices. Ethernet uses a balanced signal. That is, the data signal is sent down one wire of the pair and the exact opposite polarity signal is sent down the other wire of the pair. Any noise picked up on the way will create a spike with the same polarity on each wire. When the data arrives at the end of the cable, any signal that is matched with an exactly opposite signal on the pair is passed on as data. Any signal that matches the polarity on the other wire is discarded. The circuit that performs this filtering is called a common mode reject filter.
Twisting of paires reduces crosstalk inside the cable. Signals radiated from twisted wires will be in opposite phase from each other. When two signals of equal and opposite phase are received, they cancel each other out.
The frequency of the signal, twists per foot, and relation of where the twists occur in relation to other pairs are all critical variables. That is why kinking a UTP cable can seriously increase the crosstalk and induce errors in the data. Belden went to bonding pairs to help maintain the pair to pair geometry.
The braid of a coax cable forms a shield similar to a foil sheld in STP cable. However, there is a capacitive component between the outer shield and inner conductor with the insulation between being a dialectric. That is one of the reasons coax cable is much better than twisted pair. If you bend a coax cable too sharply, the inner conductor presses against the insulation and changes the capacitive value of the cable which is a common cause of coax cable failure.
UTP is good enough to get by, and is much cheaper to make than coax or STP. Cheap always wins over better.
