Why do electricians use#12 wire for outlets and the code allows no#14 for lighting

12 AWG can be used with a 15 or 20 A breaker. 20 A or 15 A outlets can be used here.

14 AWG must be used with a 15 A breaker. 15 A outlets must be used here.

12 AWG costs more. 20 A outlets cost more. 20 A breakers cost more. Certain areas require 20 Amp circuits particularly kitchen and bath.

Lighting is typically smaller circuits and it's getting smaller with high efficiency lighting. Ceiling fans are still low wattage loads.

I am not sure of the actual question here, but both parts of this are wrong, at least in most parts of the US.

NOT nearly all electricians use all #12 for receptacles. Some areas require all #12 (dumb rule IMO), some areas require 15A circuits for lighting.

Boat, if you post back a bit more detail about what it is you are actually asking, and why, we may be able to help you.

In our Area, we can't use #14, however, some Modulars being shipped in have #14, A Friend of Mine bought one, piss poor wiring.
Both hall 3 ways have to be on for lights to work.
When doorbell breaker is turned on, it chimes and stays energized, they wired transformer directly to chime and left doorbell wires disconnected.
Kitchen Light stays on, no switches turn it off. In the baths, 1 switch works light and fan, other switch does nothing.
Never seen so many problems in one house.

To be honest with you, if an electrician can use 14 AWG cables or individual conductors, they will do so. It is cheaper to purchase 14 AWG than 12 AWG cable.

While there are some mandated 20A circuits (kitchen counter tops) general lighting and receptacles are still calling for 15A.

If I take a "SWAG" at it I would say that more than likely the loads that receptacle outlets are being asked to carry is forcing circuit designs to change. Circuit design is outside of the purview of the NEC.

Just for example, think of the load that the receptacle(s) must service in a home theatre setup.

A 15A receptacle circuit is normally calculated to handle a load of 12A. A 20A circuit can handle a load of 16A. In general terms you only plan for an 80% load on the over-current protection device (Circuit Breaker).

Also, I prefer to use the correct receptacle for each circuit. Using a 20A receptacle on a 20A circuit, properly identifies the amperage available on the circuit. If you use a 15A receptacle on a 20A circuit, the person looking at the receptacle has no way of knowing that the circuit is 20Amps unless they go to the panel box and correctly identify the breaker serving that branch circuit.

Any way, those are just my unsolicited personal opinions on the subject.