Deploying a major release as a diff to a complex development environment is far from effortless. They probably figured the effort vs. payoff ratio didn't work out, compared to making improvements to the product itself.
Source control applications don't generally worry about dependencies, and they also generally aren't concerned with multiple projects.
Using .NET as an example, the SDK patches would include:
SQL Server (or Express or Lite or whatever they call it)
And the list goes on.
Apple's developer kit has a similarly vast suite of libraries and services, I just don't know all of their names. Each one in isolation wouldn't be all that big a deal, but combine them and you have the option of massive downloads or a nightmarish dependency management problem.
With Visual Studio for example, it's not only about merging files, but installing various DLLs (for the runtime - vcredist*.exe), COM objects, .NET stuff, etc. Also creating (if possible) proper uninstalls.
VS2008 runs some SQLserver, PDBserver, and who knows what else (run ProcExp and see)