This comes down to software patents — not on dates, but in a way that limits filenames. The only filesystem which is widely available and cross-platform is FAT, the venerable Microsoft DOS filesystem. It works on both old and new versions of Windows, worked on OS/2, works on Macs, works on Linux, and there are plenty of embedded implementations for the mini operating systems that run on cameras. It's old enough that it is public domain.

But, there's a catch. The original version only allowed filenames of eight characters plus a dot plus the three-character extension. The extensions which allow longer filenames are much more recent, and a while back Microsoft was doing some serious saber-rattling over collecting royalties. That means most cameras — and the DCIM standard — take the safe route and avoid code to generate long filenames. And that means human-meaningful date and time isn't viable, because it's just plain too long.

This is backed up by a technical manual for an embedded SD card driver, which notes:

Microsoft offers licensing for the use of its FAT filing system on a per unit sold basis. However it is generally viewed that this only applies to applications that implement the patented long file name system (LFN). It is our understanding that if long filenames are not used then no licence fee is due, however you should ascertain if you agree with this view yourself (to our knowledge Microsoft have not stated this but others have determined this based on original releases of the FAT standard by Microsoft).

Cameras or cameraphones which do write longer filenames have either paid Microsoft, are using a different filesystem, or aren't worried about the legal threat.