RAID works below the filesystem level – it doesn’t know or care what parts of the disk are “used” or not, it just sees a bunch of blocks and their mirrored counterpart for RAID1.
So it has to sync the entire disk to make sure that they match. If it didn’t, it wouldn’t know what differences are an error and which ones are just parts that the filesystem doesn’t think are used yet.
There is a --assume-clean flag you can use in mdadm to tell it not to do that – but you should only do that if you are certain that the disks contains nothing but zeros. And I think it only works for RAID1, not for RAID5/6.