It is necessary to prevent Genesis Block overwriting. Satoshi timestamped Genesis Block with the NY Times message to prove that this block was not mined earlier. In old versions, Genesis Block was not any special case in code. You just started from zero hash and built on top of it. But then, everyone could overwrite the whole chain, including the Genesis Block. And later, Satoshi hardcoded Genesis Block data in the client to prevent that kind of attack. But he forgot about adding that transaction to the database, so it became unspendable.
So, you can start with no Genesis Block at all, just build on top of zero hash and you will get simpler code. But then, overwriting all blocks will be possible, including the first block in the chain.
With a new blockchain started in 2021, even if the genesis block is precomputed with a non-zero difficulty, it may still be feasible for someone with considerable mining power to just compute the first X blocks again and replace the block history, if the incentive for doing so is large enough. So, I doubt that the risk of overwriting blocks can be fully eliminated just by having a precomputed genesis block. It may be a part of an effective strategy, though.