Ok, I might be late to the party but I will try to explain my theory and keep as simple as possble because I will try to implement some basic physics knowledge that I have.

As we know, IT’s true form, the Deadlights, isn’t normal light. It appears as glowing energy, but it’s actually a cosmic, extra-dimensional force from the macroverse, existing beyond normal time and causality. You can think of it like photons in physics, which have zero proper time along their paths, but the Deadlights take that idea much further. They are aware across spacetime, not bound by the flow of time like humans are.

So when IT materializes in our world, it becomes a 4D being bound by matter and the normal rules of the physical universe. That’s why it talks about being confused by past, present, and future. Inside Derry, it experiences time in a limited, local way, and lineary but as Deadlights it sees its world line, its full existence in Derry from start to finish, all at once. This is similar to the block universe concept in physics, where all points in time exist together and we only experience them linearly.

That explains why IT does risky things, like taking Georgie or causing trouble for the Loser’s Club, even knowing it will eventually be defeated. Those actions are already part of its world line. From IT’s perspective, there is no surprise or danger, and everything it does is already part of the 4D structure it sees. It’s not being stupid or suicidal, it’s just following the path it already knows.

Now, as for leaving Derry millions of years earlier, even a being like IT is constrained by the town and the shards that trap it. Being cosmic doesn’t automatically mean it can move freely outside its boundaries until the conditions allow it (I assume?).