Too many kids on the field to lose control of the game. The biggest reason that your easy advice isn't followed is because of the extreme nature of the necessary actions that fifa refs must employ to consistently manage a game. Please don't change this discussion to a debate the relative importance of the 11-10 game. Thus, most competent refs withold the liberal use early, of the much needed caution, that provides the foundation that you seek. It also puts the ref on the spot, and forces him to often go against his word unless he dillegently follows up on his threats.
Most refs just keep on threatening.
Many players laugh about these threats, and then take the challenge and find out how far they can go before a card actully surfaces, when in fact, it should have surfaced already, instead of the verbal taking place. This scene, taken in it's entireity explains why so many games get out of hand in soccer. The other sports also have challenging games, but not to the extent and certainly not at the same frequency. The wild ones are big news in football, basketball, and baseball, but are routine in soccer. This option takes the severity out of the first yellow and therefore adds a very important game management tool to the officials early tactics.