The Afghan security forces have a very high desertion rate. In the last year, it was 25 percent for the police, and 18 percent for the army. In contrast, the U.S. Army desertion rate over the last decade has averaged a bit less than a half a percent a year. That's very low, even by American standards. During the Vietnam war, the rate varied between 1.5 and 5.2 percent a year. During World War II, it peaked at 6.3 percent in 1944. West European nations have slightly lower desertion rates, partly because fewer European troops ever get to a combat zone, and in some countries, you can buy your way out (if you volunteered). In most nations that still have conscripts, the draftees cannot be sent overseas. The American desertion rate plummeted when conscription ended in 1972. The Afghan security forces are all-volunteer, but the police and army leadership is often corrupt and inept, and this is the major cause of desertion..
The high Afghan rates are typical of poor countries that also suffer from some internal violence. You'd think that a nation with a high unemployment rate of 40 percent, there would be no shortage of recruits, and few would desert. But that unemployment rate is calculated by Western standards. In Afghanistan, it's been like that for centuries. A lot of men don't have regular jobs, but they belong to clans and tribes where everyone gets taken care of, sort of. Keep in mind that Afghanistan is the poorest nation in Eurasia. Expectations are different. The really ambitious either find work, or emigrate.
Joining the army or police is actually an alien concept in Afghanistan, for the nation has never had a national police force before, and the army, in the past, was always a small force that guarded the king. The real "army" was volunteer contingents from the tribes, who served as long as they were needed, got to keep whatever loot they could collect, and went home as soon as possible after the war was over.
The current army and police force don't pay as well as the drug gangs and their Taliban allies. There are even warlords who pay better, and do a lot less dangerous fighting. With the gangs and Taliban, you get hired by the day, and can walk away at any time. Only the Western style army and police expect you to stay for a fixed amount of time. That a pretty alien concept in Afghanistan (where 80 percent of the population can't read an enlistment contract, much less sign their name to one.)
The desertion rates have been coming down (from a peak of 40 percent), and a recent pay increase, and the increasing supply of trained officers and NCOs, are continuing to lower it. A major problem with the training of police and army leadership, is the shortage of foreign trainers. Training is a very demanding job, and there has been more difficulty getting trainers from foreign nations, than getting combat troops.
After eight years of effort, the Afghan Army has 98,000 troops (expanding to 170,000 by next year, mainly because there are enough trained Afghan troops to do most basic troops training with Afghans). The police force stands at 90,000, growing to 134,000 by next year. Both these growth estimates assume desertions will remain high, by Western standards.