The being late to the party but having the best implementation reputation might be behind this, but overall Apple tend to be very conservative in what they release, even if they might be doing similar experimentation to Samsung behind closed doors. Yes this has led to a product line that looks very staid in comparison to the weird and wonderful contraptions other manufacturers are beginning to release, but exotic form factors are probably really only of interest to a relatively small number of tech enthusiasts at the moment, so I doubt they would get past Apple's assertive bean counters (Samsung apparently makes a loss on their folding phones currently, I doubt that would be something Apple would even consider).
Nokia always had a reputation for putting out some crazy designs, but ultimately that didn't help them when the core business came under attack from Apple and Android - Apple just focus their R&D cash where the return is, if the folding/ flip market takes off and that shifts the centre of economic gravity, they will likely follow the money - if they're smart they will be ready with a design to roll out in short order if that happens.
tl;dr - Apple have followed more of a reactive than a proactive path (outside of specific keystone features) since releasing the iPhone 2G, but that doesn't necessarily mean they aren't looking into new tech like other manufacturers, just unseen.
