In layman terms, because no one trusts anyone by default

But it is assumed that if you are using Bitcoin, you are trusting the Bitcoin network. Namely, the consensus or overall opinion of the network regarding particular transaction. If it gets confirmed by the network, it is considered as legitimate and binding to everyone. For example, when you send 1 bitcoin to someone from your wallet, the confirmation of this transaction means that now you have 1 bitcoin less in your wallet, while the receiving end (the wallet where you send this bitcoin), consequently, has 1 bitcoin more. Without confirmation, your transaction is essentially neither here nor there