If only it were that easy. The problems with cross-platform development are myriad. The LCD experience has already been outlined, so here are the others:
1) It's write once, TEST everywhere, and you can't debug the code you actually wrote -- only the specific translation of that code into native code. And that sucks beyond words. It can be incredibly time consuming to the point where it easily erases any time you saved in development. And the longer the lifecycle, the more of your budget this is going to consume.
2) On a related note, profiling and performance tuning is a bear. If you do anything that requires performance, cross-platform is the wrong way to do it.
3) The potential for bugs is twice as high, because now you don't just have to worry about bugs in the native SDK, but also in the abstraction as well. While the OEM can generally afford to test the hell out of their SDKs, cross-platform suites have much less resources at their disposal. And it shows.
4) You are always playing catchup. New features take longer to implement, because you have to wait for the third party to either implement the new feature, or to implement it right.
5) You're needlessly adding another layer of dependence. None of the "popular" third party platforms are anywhere close to guaranteed that they'll be around in a month, or 6 months, let alone 2 years from now. If your favorite library goes away, you find a new one and make a few changes. If your entire SDK goes away, you're fscked.
If your software doesn't matter at all, if it's just a hobby, has no business case, and you have no plans (or potential) for going commercial, then by all means, use whatever development tool strikes your fancy. Personally, I would never recommend that anyone use cross-platform tools for anything they plan to support indefinitely. Even for a prototype or an MVP -- too often the prototype becomes the codebase for the final software, despite prior assurances to the contrary. Don't waste effort developing something you can't build on, unless you have no other choice.