The God Hand are not physical beings and cannot step into the real world. They can still influence events in the real world, just never actually step foot in it.
They only appear during an Eclipse (even then they are not actually in the real world, just that middle area between worlds).
In order for Griffith to come into the world he needed a physical form and a ceremony must take place. The particular ceremony where Griffith is born into this world is actually a particularly rare Eclipse. Even more rare than the one where he invoked the Crimson Behelit to become Femto.
So in summary there are three invocations/ceremonies:
1. A normal Behelit is invoked and a human sacrifices something dear to them to become an Apostle.
2. A crimson Behelit is invoked and a new God Hand is born. Griffith becoming the 5th God Hand. Requires lots of sacrifices.
3. The Reincarnation (rare) where a God Hand comes back into the physical world. Requires lots of sacrifices. Femto is now Griffith again. But they are still the same person, just now given physical form and just as powerful.
You will see in the next season the results of this ceremony and the effect it has on the world of Berserk. Also, this Reincarnation ceremony is all the more juicy due to the relationships between the characters and the vessel that Griffith/Femto uses to hatch into this world.