There are two answers to that. One is that to fully support these rich formats you have to have compete feature parity with Office, which isn't trivial. The other is that we're comparing to plain text, which doesn't even pretend to give you any control over how things look.
The alternative offered is Markdown. How can you say that it's ok for Humans to revert to raw Markdown, but unacceptable to open a legacy .ppt format in an application that may not render with 100% fidelity?
No one ever said that humans need to write raw Markdown. Take a look at Typora as an example Markdown editor. Raw Markdown would be the fallback scenario, which is still readable/grok-able. docx/pptx/xlsx don't have such a fallback scenario.
There are countless software packages that can edit and display Office XML files besides just the Office package, so I'd think the fallback scenario is using one of those.
