Because they have a mental model of what is inside the previous 6 months and can make a judgement regarding whether or not they are comfortable revealing that i

Because they have a mental model of what is inside the previous 6 months and can make a judgement regarding whether or not they are comfortable revealing that information, whereas the future 6 months could include purchases that they may not want to reveal for various reasons. I also think many consumers would simply be creeped out by the idea that these companies can continue to maintain access to their bank statement for 6 months into the future, especially in cases where the consumer has a dispute or negative experience with the company. There are also some underwriting arrangements where companies could leverage future Plaid data to make decisions about how to treat a customer (e.g. monitoring bank balances so that rebilling a delinquent customer can be automatically rescheduled after a deposit)