Because the Bitcoin network is a network of peers who have connected to one another.

Satoshi had the first block mined and then he stopped mining until someone else started.

I remember reading somewhere that it was as if Satoshi had some kind of mining advantage in those early days and if I remember correctly I think his hash power would turn off during the first 10 ish minutes after each discovered block to allow the other miners to get their chance to discover blocks as well.

I don't believe this was to get Bitcoin, but to prevent early 51% attacks. He balanced out the playing field until there was sufficient decentralization of hash power, then let the network do its job without him

This is the main reason why he has such a big stack that was never touched. Securing the network in the early days long enough for it to be able to survive without him.

This article contains some interesting information, including a pretty neat pie chart that kind of supports what I was saying in my original comment.

Satoshi mined a mass majority of the coins while their were very few players out there also mining... A time where the network was at its most vulnerable. He a centralized force on the network ensuring its integrity until the network had spread and sufficiently decentralized.

He was probably cheating the hash power / difficulty adjustment by flicking his hash power power only on around that 10 minute mark so his OP hash power wasn't ripping the difficulty up as other curious cypherpunks and such began running their Bitcoin core with the hashing power of a single ~2008 CPU.

All to secure the network. It wasn't in a trustless state at that time, and he could only trust himself until there was sufficient evidence of decentralization, then he basically stopped altogether.

So, Satoshi mined his coins for no economic purposes. He had a lot of hash power but would only use it around the ~10 minute mark to not disturb the difficulty adjustment to far in his favour that others could not discover blocks. When the network was decentralized, Satoshi stopped mining altogether as his goal was complete.

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Satoshi was trying to setup Xdebug to work with phpStorm.