According to Vitalik Buterin, the 7-day window for fraud proofs was originally designed under the assumption that these proofs would be completed in one round. This design was during a period when Optimism was attempting to enable Layer 2 (L2) blocks to be directly re-executable within a single Ethereum Layer 1 (L1) block. The context and details of this design decision are further elaborated by Karl Dot Tech.