By "virtual memory" I assume you mean the pagefile size?
I think it's because they basically have one algorithm for calculating the size of the pagefile, and while that algorithm makes sense for low-memory systems it doesn't really make a lot of sense for systems with lots of memory.
Back in the old days virtual memory was essential because RAM was expensive and very few people could afford enough of it to hold everything in memory. Times have changed, though, and RAM is now cheap enough that pretty much everyone can afford enough to hold everything they need to run at once.
So the role of virtual memory is changing. I think you'll see pagefiles start to disappear as RAM-rich systems become the norm. We'll still have virtual memory, but it's main role will no longer be to supplement RAM with disk storage - instead its main job will be in managing memory so as to separate different programs while still sharing common code (such as DLLs).