You never know when they might falter one quarter, with so much riding on so few products. If there's another Foxconn flaw in the hardware for the next iphone or ipad, for example, or what if Steve Jobs leaves, for example, who knows. Labor in China is getting more expensive, too, so Foxconn is looking to India and Vietnam, and they've already complained about how 'difficult' the iphone 5 is to build. Of course a bad quarter still won't seriously damage Apple since it has so much cash in reserve.
There is no rule that subsequent quarter revenue should be higher than the prior quarter. [For e.g. the holiday quarter is generally the highest revenue yielding one for consumer electronics companies]. That said, Apple generally sets the expectations way low. The street knows that. They figure out a higher number (based on the past) and make that their expectation and price the stock as such. The interesting thing though is that Apple has been beating that higher expectation as well.
I'm sure not going to buy an iPhone or iPad right now, given the heavily-rumored launch of the next-gen models in late Q3. That could be why they're sandbagging the revenue projections.