Because the data may not have replicated to the other nodes yet. As the parent comment noted, you could synchronously write to all replica nodes, but thats a big performance penalty and not partition tolerant.
They pay the penalty in latency, not in throughput. That's like no performance penalty at all if you work asynchronously. They make sure that the data cannot become inconsistent between nodes by ordering the transactions. The penalty that comes from this is that latency can become high, i.e. the answer time for individual transactions. While maintaining a maximized throughput of millions of transactions a second. It's like an internal 'pumping rhythm' with tightly synched timestamps.