Right now the nearest sand to me is 3 miles away at Lowes.
Unless you live on bedrock, there's sand right under you. If you only count the beaches, there will be much less than 7500000000000000000 grains of sand, making it even more (lol) likely to get the exact same grain of sand.
Once I get there 60 minutes later I did zero tries and the super computers did 60 billion seeds..
Yeah, but what's 45370982256125128461 divided by 60 billion? That's still 756183037 times less likely to find the same seed than it is to find the same grain of sand.
Instead of sand, how's this?
I like this analogy:
Imagine a massive wall of lockers. Each locker is 1mm by 1mm, and the entire wall of lockers is a square 2 light years on each side. When you choose a private key, you pick one of these lockers at random. When someone sends you bitcoins, there's some magical inbox which puts the bitcoins into your locker without telling the sender anything about the location of your locker.
The lockers don't have locks. If someone knew the location of your locker (ie. your private key), then they could just go take what's in it. Similarly, it's possible to choose a locker at random and find that someone has used it already at some point in the past. But there are just so many lockers that in reality it's never going to happen, even if humanity devotes all of its efforts to searching through all of the lockers.