where is this perception that proper cross platform development is an unsolved problem coming from? well written software has a platform layer which is usually small unless your product is trivial.
you also don't have to pay through the teeth with towering technology stacks.
maybe its connected to the fear of low level programming, or the fact that Google seem to not like using the established cross platform technologies, and made some pretty questionable decisions about the android development platform - like not giving a C/C++ compiler through the NDK for a long time despite the fact that everything they did was built on a foundation made with one so they obviously had it.
maybe they need a lot of platform specific code, but tbh i find that 70% a very unimpressive number. i would not even be impressed by 95%, i'd just call that 'normal'.
maybe this makes more sense of you are constrained to rapid application development, but given my experience with that a lot of the rational there is fallacious... i don't think i could build a cross platform app in a week without dropping down to C++/ObjC/Java and minimising the amount of Obj-C and Java as far as possible.
i suppose an important argument is that targetting the web with server side native stuff rather than client side java scripting things is a bit of a nightmare and lots of people don't even realise its an option...