Living in a foxhole for a week isn't exactly conducive to keeping long hair free of microorganisms and small critters. Although these days, first nation and female soldiers are allowed to braid their hair, so things have progressed.
This!
I will also add , while living with 60 or so , in the same building, a military haircut is conducive to seeing parasite infestation, is cooler in high heat situations, and won't get into your eyes.
In hand to hand combat, long hair is a weakness.....in a knockdown fist fight, if you have long hair and I get one hand into it ( and trust me , I will ) a very large man can be knocked off balance with ease, and that head with the long hair can be forced into my other hand, my knee , or more likely a hard object like a post ,chair or whatever else nearby.
For those not familiar with it a very simple example is to stand with your feet shoulder width apart, and move a shoulder.
You can very easily keep your balance without moving your feet....now move your head 8 inches to one side.
You must move your feet to avoid not falling....once your opponents balance is compromised, the action of falling is enhanced two fold.
Keep in mind a opponent can still fight with a broken hand, but all life travels through the neck into the head.
Nerves, blood and oxygen and reflexes are there.....I am 190 to 195 depending on the time of year.
I have taken down men that outweigh me by 60 pounds, and much younger in seconds, by grabbing their hair and bringing up my knee ,hand or elbow, into the neck, or temple.
Plus chicks dig a high and tight haircut....as Miss Erica will tell you.
There is also a physiological aspect to removing your long hair in basic training....everyone is the same, your individuality is taken away, to be broken down and molded into what they consider .....part of the machine that will be sent into conflict.
