Because before now I knew that the genesis block isn't spendable but I really didn't know the reason why.
The main reason why the 50
BTC of Genesis block's coinbase coins aren't spendable is what ranochigo wrote: they aren't part of the UTXO set. So, why aren't those 50
BTC in the UTXO set? I'm not sure if I found a good explanation, see below quote (only did a very brief search as I didn't quite remember the answer myself).
Genesis Block Coinbase Transaction: In Bitcoin, the genesis block is the first block of the blockchain. Notably, the coinbase transaction of this block is hardcoded into the Bitcoin client. This transaction is never added to the Unspent Transaction Output (UTXO) set, making its output unspendable
I'm not satisfied with this quote, as I don't see the reason why this particular UTXO isn't "parsed" and added to the UTXO set, even when the Genesis block is hardcoded in the Bitcoin node client. I remember vaguely another more reasonable explanation but can't find it atm.
To my knowledge this special case of the Genesis block coinbase UTXO is unique to it.