When I touch the live wire from a power socket with my bare foot on the floor, how does the circuit close (how does the current return to the supply)?
Current will flow according to Ohm's law - I = V/R.
The following is approximate and will vary with user , contact points and more, but:
You may be able to "feel" current in the 10's of uA (microamps) range,
Things start to get painful in the 100 uA - 1 mA range,
You will feel a "full shock in the 1 mA - 10 mA - up range.
Anything over 10 mA for more than a fraction of a mains cycle MAY be lethal depending on the current path through your body.
Worst case, currents lower than you expect may kill you.
So, to get a significant shock say you require around 1 mA.
At 230 VAC that's a circuit resistance (or impedance) of about 250 kOhm.
In practice you will find that feet to true ground resistances of this resistance or lower are "quite commonly achieved". Partially this can be due to capacitive coupling effects in the conduction path, massively parallel paths to ground and just plain bad luck.
A 0.01 uF capacitor to ground will pass about 1 mA at 230 VAC. When you work out the distributed capacitance by all paths to ground, the various parallel resistance paths and more, it's not hard to get currents of this order. I base this not mainly on calculation but on the fact that this is what happens in practice. Theory and calculation WILL explain this if you apply them, but experience and common sense demonstrate this is what causes the result.
First of all, how do tiles in the house floor conduct?
As above - some moisture, some capacitance, massively parallel paths to ground from the contact point, ... .
Then why are any other insulating materials not usually used for flooring?
For much the same reason that most people do not live their lives wrapped in foam padding, live mainly in bed or drive cars full of shock absorbing foam. Floors are made for walking on. They are not primarily intended as a part of an electrical safety system. In critical situations special attention may be given to such aspects but in most cases there are better, simpler, cheaper and more logical ways of providing electrical protection.