More than 20 years after the invention of the IBM PC (and the Apple Macintosh, for that matter), why is it still so difficult to build robust applications?

To assure an enhanced standard of living? If it was easy - every one would be doing it. Our profession would be relegated to "skilled labor".

Building robust applications is not so much a fuction of "tools" as it is of those so called professional ideals: concept, planning and architecture.

MS would have developers migrate to .Net. On top of learning a new language (for pretty much everyone except C++ programmers), learning a large class "framework" seems to be required.

Mom and pop are ninety percent of the market. They don't want expensive client/server architectures and licensing.

"Mere Mortals for .Net" framework;
There are negatives associated with off the shelf frameworks. To many of us, understanding how the underlying architecture works is important. Commercial frameworks seem to have a tendacy to hide that from us. We become framework code scripters. To me that is a step down from programmer or developer.

LAMP (Linux/Apache/MySQL/Perl [or PHP or Python]) seems increasingly popular but my impression is it's mostly for Web based apps, may not be too suitable for desktop apps. Technology is relatively new and tools not as advanced as MS or Java.
I have connected VFP to Linux/MySQL servers. The data comes back looking just like SQL, Access or a free table app. Linux servers are not just for internet apps.

I like that VFP has multiple 3rd party frameworks available. Their judicious use can really boost productivity. But, for those cases where technology matters, it's not mainstream.
Some of those "frame work" projects are in trouble. Again, it may be because framework coders don't have the benefit of of a rudimentary understanding of software development. When I see a framework ad, it says to me:"Too stupid to write software, buy me!" I could be wrong - the last framework I saw demoed created a 2 MB exe for just one form!

Why does it seem that development is getting more complex rather than easier?
The cream rises to the top, thats were the big bucks are:-)