Why is the fuel:air ratio of a turbojet engine not around ~1:15 (the stoichiometric ratio)?
Edit: to honor my original answer, I shall keep it as it is. The question was edited so much I think it would have constituted a whole new question. In it's original form, the Q presented jet engine f/a ratio as 1/4 - 1/3 and compared it to the stoich ratio of 1/15.
Your numbers seem way off, except for the stoichiometric fuel/air ratio (for kerosene).
Please see the following NASA article: Beginner's Guide to Propulsion and EngineSim
Fuel and Air Relationships
Activity
A quote from there:

For jet engine combustion, the important parameter is the fuel/air ratio (f/a) which is the ratio of the mass of fuel to the mass of air being burned in the engine.

The stoichiometric ratio of 1/15 can also be written as approximately 0.067
The article further elaborates on the subject:

For example, at sea level an airliner taking off at 375 mph with the throttle at 100% has an f/a of .017

This 0.017 translates into about 1/59
No matter how you look at the ratio, mass or volume, in no case will a jet engine run at f/a of 1/4~1/3. The mixture will simply be too rich to ignite and burn.
Here is a link to a Wikipedia graph about the range of typical jet engine air:fuel ratios:
Combustion stability limits of aircraft gas turbine.svg
Please note the graph displays the ratio in reverse order from what was used before in this answer, ranging from ~25/1 to ~180/1.