I would ask the same question. However, that's not how patent law works in the USA. Nokia doesn't have to spend their time/money watching other people's products, tearing them apart, and looking for patent infringement. All Nokia needs to do is SOME DAY realize their patent was infringed and then file a lawsuit. That's it.
But I am sure that Nokia knew a very long time ago that Apple was infringing and did nothing (which is perfectly their right)...and let Apple make billions...then file a lawsuit to cash in. Think about it...why would Nokia file suit immediately to earn, say, a pesky $5 or $20 million (and give Apple a chance to make the product better) when Nokia can wait years and earn hundreds of millions or billions? It's the waiting game that ALL patent holders play.