Why must generated coins mature 120 blocks before they can be spent? I mine bitcoin testnet blocks and I find some blocks with difficulty 1.

Some examples: I mined these blocks:


Block 1
Block 2


Why must generated coins mature 120 blocks before they can be spent?
This rule is to prevent people from unintentionally spending Bitcoins that they wind up not owning.

For any Bitcoins other than the block reward, unless you intentionally double spend, a reorganization leaves your transactions valid. So they can just be included in the new blocks. (Assuming they don't spend the proceeds of a double spend.)

However, if there's a chain reorganization, there's no way to recover block rewards. Whoever mined the blocks in the other chain gets that reward. That would make your transaction invalid since it would spend funds you don't actually have.