This may actually take longer than having a line item for each file, consider:
> Visual Studio, please open all *.cs files in this directory (recursive)
vs.
> Visual Studio, open all of these files
Also, by itemizing each file you can exclude single files from build easier.
This is all speculation.
EDIT: Also, I think they did this because they didn't expect users to be mucking around in .csproj files.