UNIX tools can focus on solving a single problem well because they interoperate between themselves, so you can join tools that solve different problems and make something useful out of it.
SaaS has no such option. It either does every little thing you need, or it doesn't solve the problem.
They're ultimately limited to which SaaS lets them, which is usually not much. For many, deep interoperability goes against the core objective of "engagement" (i.e. trapping the user in the service for as long as possible).
You have a single filesystem, so maybe you can open the same document in both one after another to work on the parts that need D and the parts that need C. Of course this relies on having a common format, but in the local-apps world we mostly have those, whereas in the SaaS world we don't even have a filesystem to share.
SaaS has no such option. It either does every little thing you need, or it doesn't solve the problem.