If I started a savings account in your name from the day you were born and just let it rack up money, then you would have a very large sum by the time you were 30. That money is worthless to you however, if you have no idea that it exists.
They can't use the tech right off the bat because they don't know about them. There isn't a Sharingan/Rinnegan instruction manual. It was never explained how a MS user discovers their unique ability (since each pair of MS has a different trait), but it can be reasonably assumed that when the user activates MS it jumpstarts that ability, granting the user knowledge of its existence. kakashi is not an Uchiha, so even though he has access to the MS from the start, he had to work up an insane amount of chakra control to activate it (somewhere between part 1 and part 2).
Rinnegan users are different, since the Rinnegan gives the user access to any and all tech, its just a matter of training.
