Because Blizzard literally told us it would keep them.
Q: When creating tier set items with the Revival Catalyst, what is the item level of the tier piece after the process is complete? Is it the same item level as the base piece or are there item level ranges that snap to LFR, Normal, Heroic, and Mythic item levels for tier?
A: The resulting item will have the same item level of the item that you put into the catalyst. It also keeps Mythic/PvP upgrade paths, tertiary stats (leech, speed, etc.), and sockets.