Quick answer: Because they didn't entered our event horizon. Some never will. And some will move out of our event horizon - their last photons that'll be received here being sent right now.
Let's do some fact checks first:
[...]the galaxies that are now at the 'edge' (not visible
theoretically) must have been (at some point in time) at place around
where the Earth is at now[...]
According to current calculations, the first galaxies may have formed around 200 million years after the Big Bang - older estimates went with the 400-500 MY range. For a long, long while, there were no stars to be seen. So if you go back in time you won't be seeing the same structures we see today.
Second, and that may be an awkward mental experiment, nothing else was occupying our place other than ourselves. I'll ask you to excuse the cliché, but the old balloon example is very apt to explain this:
As the universe expands, the distance between celestial bodies increases. Now, here's a way to put it: space is being generated in between the objects.
And not only that - while there's a limit on how fast you can move on the balloon's surface, it doesn't apply to the amount of space being generated.
As a direct consequence there's a bubble around us that basically works in the very same way as a black hole's event horizon does:
- In a black hole, gravity is so strong that its pull on photons exceeds C (the speed of light, or 299,792,458 m/s);
- On a fringe object, the amount of space generated by the expansion, per second, may exceed C; its photons will never reach Earth.
And it seems that we live in a accelerated universe - that is, the spatial acceleration ratio is actually going up. If that's correct, some fringe objects will slip away form our event horizon, disappearing (from our point of view) into a sample of the heat death of the universe.
Scary, huh?
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