Cable Splitter Loss and Impedance Matching
Resistors are about the least efficient way to build a splitter. A resistive splitter will split the current and waste the excess voltage. So for a 2-port resistive splitter you lose 6dB. They do have some advantages in specialist situations but practically they aren't used much (I found when I needed one I had to build it myself).

Afaict low loss TV splitters are based around transformers. The transformers can match the impedances without too much loss and can maintain reasonably consistent behaviour over a very wide frequency band. A transformer based splitter built out of ideal components would have a 3dB loss but real transformers are not ideal, so 3.5dB is about as good as you can get. 

Mini circuits (a maker of high quality RF splitters) have an appnote describing how transformer based splittlers work. https://www.minicircuits.com/app/AN10-006.pdf 

Transmission line or inductor/capacitor based splitters (like the Wilkinson one mentioned in another answer) can be even more efficient (real transmission lines and simple components have less loss than real transformers) but they only work over a narrow frequency band so are not suitable for TV (there is approximately a factor of 2 frequency difference between the top and bottom of the UHF TV band, far more if you include VHF).