Why can't you just copy-paste digital money?

Because something has to check that each coin hasn't already been spent. Copying a digital file costs nothing, so without that check you could pay with the same money twice, which is called double-spending: the unauthorized spending of the same money, digital or conventional, more than once. Like counterfeiting, it creates copied currency that didn't exist before, which can devalue the currency and erode people's trust in it. Most systems rely on a trusted referee to do the checking; Bitcoin replaced the referee with a network that agrees on one shared ledger.

The easy fix is a referee: prevention of double-spending is usually implemented using an online central trusted third party, like a bank, that checks whether a token has already been spent. The catch is that this referee becomes a single point of failure — if it goes down, is hacked, or is corrupted, the whole system is stuck.

Remove that referee and the problem gets much harder. In a decentralized system, many clients must instead store compatible copies of a public transaction ledger, and consensus protocols let them agree on which version of that ledger — the canonical chain — is the real one. Bitcoin's version of this, launched in early 2009, batches transactions into a chain of blocks, and each new block on top of a payment makes it more costly to overturn, but it isn't foolproof: anyone who doesn't wait for enough confirmations risks having their incoming payment reversed if the chain briefly reorganizes, and anyone who controls over half the network's computing power can eventually force their own version of history to win.

This isn't theoretical. In March 2013, a bug in the Bitcoin software caused the chain to split; a merchant had already confirmed a $10,000 deposit when miners reverted 24 blocks, and the customer, as an experiment, spent the same bitcoin again on the older chain. Years later, the Bitcoin fork Bitcoin Gold lost exchanges $18 million in a single 51%-attack double-spend in September 2018, then $72,000 more in January 2020 : real money lost to the very risk that consensus protocols are meant to reduce.