That's largely the fault of Intel, since the notebook-class CPU chipsets do not support the low-power RAM in configurations above 16GB. Apple could have used desktop-class chipsets and RAM, but then battery life (already criticized) would have taken a hit. The rumor is that Apple might release one 32GB configuration later this year.
That said, even most "pros" don't need more than 16GB. It's mainly useful when doing hard core video editing or multiple virtual machines. With fast SSDs paging is far less noticeable than it used to be.
Which 'pros' don't need more than 16gb of RAM?
Anyone working with video or fx does.
Anyone working with large PSDs or high-resolution photos in Lightroom's does.
Anyone working with audio does.
Any developer running multiple VMs does.
This whole line is complete bs. When people talk about pros they don't mean an office manager who uses his MBP to run PowerPoint.