1. Because SPICE has no knowledge of physical size, layout, position, finite speed of light, etc.
a. A simulation is entirely instantaneous, approximated with poles and zeroes (and one pure delay element: a transmission line -- which you don't usually use because it tends to slow down the simulation).
b. There's no good way to introduce this property, either. The most representative format would be a matrix of overlapping transmission lines, of various lengths. The matrix represents the delay of individual signals, and the crosstalk (coupling) between adjacent signals. Different impedances of transmission lines suggest inductive or capacitive character, at least at lower frequencies. Needless to say, an NxN matrix (for N nodes in the circuit) of random length/impedance transmission lines will run extremely slowly indeed, if it runs at all; let alone trying to come up with all that data!
You certainly wouldn't want an environment where random line lengths are added arbitrarily into your circuit! I think I've seen, before, simulators where the graphical length of a "wire" adds resistance... lord help them...
c. There is only one reason why "capacitance" and "inductance" exist at all: because they are convenient approximations of transmission line properties. Everything in reality has to pass through space at the speed of light (or some fraction of it), and therefore transmission line effects are both fundamental to reality, and critical for the complete analysis of a circuit. But for a partial analysis, for frequencies where the delay is a small fraction of the cycle, we can approximate the speed of light as infinite and, lo and behold, transmission lines look like an LC circuit!
2. What is SPICE?
SPICE is a numerical solver.
It does not know the real world, or physics. It contains some useful passive and semiconductor primitives (betraying its origin as a tool for IC development, where assuming the speed of light is infinite had historically been an excellent assumption), and that's about it.
It is up to you, the modeler, to create a system which is representative of the tiny slice of reality you wish to simulate.
3. Don't know what to put in? Try everything! Fail, fail hard and fail often! Build real circuits! Make horrible layouts on the solderless breadboard. Make good layouts on the solderless breadboard! Yes, there is such a thing; I've built ~1MHz switchers that way before. It's not really all that hard, once you've learned how to spot a current loop path between signal and ground (and, by the way, remember that there's no such thing as ground -- voltage is only ever a difference, and "ground" is a real physical conductor like any other in the circuit -- and unlike SPICE nodes, which are perfectly conductive to all pins on the node!). Build circuits on ground plane, dead bug style or whatever. Make a few PCBs; measure the ground loop voltages yourself! Get used to the idea of measuring funny (RF) voltages at different points along an otherwise solid conductor.
Read textbooks, do the homework problems as well as you can; or take full classes on the subject! You can develop an intuitive understanding of fields and layout parasitics, but it will take a lot of time, and having a quantitative grounding is so much help.
Tim