unless you have an overt system, alters can be more hidden not only from you but others around them. hiding and blending in became a way to protect you, them, and the whole system.

if i would not have told my therapist about the others (who never saw or knowingly talked to an alter in the years i've been seeing her), i wouldn't have the diagnosis even though she suspected i had DID or OSDD (which are similar with minor differences). i don't have amnesia between alters or they do not come out and live their own lives, etc. which 'disqualifies' me from an actual DID diagnosis, from my understanding. but i do have distinct alters as well as others that might be considered more fragments or not as fully developed. the treatment is still the same though.

the others have been hidden from me to varying degrees, except i do recall several of them in childhood, though many more appeared in my teen years and over the years since.

so, to answer the question, they hide to protect you and themselves, the whole system, etc. they don't always want others to know they are there unless there is a reason for it. they usually will come around when they feel the time is right, they get triggered, or they feel safe to make themselves known.