Because that's what's printed on your FP. The FP doesn't say come back between 1:30 and when ever you feel like it.
Okay, I have found a post from Knox, The Canadian Guy, Moderator, from one of our many other Fastpass discussions and I think it gives a pretty good explanation as to the effects of late fastpass return:
(Quote)I read a very interesting internet article musing on the Fastpass system that analyzed how it works from someone with a background in economics and that whole "supply-demand" thing... he actually looked at this all from an scientific perspective... He used big words and talked about the 'supply-curve' and such. Of course I can't FIND it now that I need it.
In the article, he indicated that something like fifty percent of all fastpasses were used within their listed time window. Thirty percent were used "late" and twenty percent were never used at all. If what's being reported here is true, that I suspect those percentages might have shifted.
He further explained that using a Fastpass late really wouldn't affect the system because if you didn't use your FP during the alloted 1 hour window, then that meant that at least one rider from the standby or fastpass line (or depending on the size of your party - BOTH) got to ride "sooner" than they would have, had you been there to take your spot. And then everyone in line BEHIND them got to ride sooner too.
It's a zero sum game. It's worth noting that they don't send around an empty ride vehicle just because you didn't show up.(You're not royalty you know.. only Cinderella gets that treatment.)
So.. regardless if you show up or not, the ride still runs at full capacity.
Therefore.... for example, if your family of five wasn't there between 11:05am and 12:05pm .. then there are five people who FOR SURE got to ride sooner than you did.. and an untold number of people who were behind them in line who ALSO got to ride sooner than they would have otherwise... up til the moment you arrive to redeem your FP.
Another way to think of it is this .. Think of the queues like a clock... If you aren't there to fill your spot, the clock starts running fast. We don't perceive it as running fast because we don't see the numbers to tell us that "X" number of FP holders SHOULD have redeemed their rides by now but haven't. When a FP is used late, the clock starts to correct itself.
To the untrained eye standing impatiently in the stand-by line -- it might appear that those darned late-arriving FP users have delayed their riding... when in reality the standby folks would have likely ended up on the ride at the exact same time had the people used their FP's within the alloted window -- they just would have PERCEIVED it differently.
When you followed the math to its logical conclusion -- the Fastpass system doesn't really break down unless every FP holder on a given day wants to use their FP's at the exact same 15 minute window at the exact same attraction.
Perhaps that has been happening around meal times, parades and park closings?
Disney had made no secret of their liberal policy on return times.. so much much so that I think that the change to a firm return time policy might be very problematic for them in the near term.
Thanks.
Knox (End Quote)