Mammal hair naturally grows in cycles. Each individual follicle will grow a hair for a certain time (and as a result length) then pause in growth until finally shedding that hair and beginning to grow another. The follicles are not all synchronized in this cycle though so you won’t see all the hair shedding at once.
For humans some hair has a relatively quick turnover such as arm hair which is why it never really gets super long. Head hair though has much longer between the shedding cycles of the individual follicles so it can get quite long if not trimmed.
For animals the length of this shedding cycle largely determines the length of their coat, and it can be varied over the year so for example during the winter the cycle can be slowed so their net length of fur increases.
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