I think people underestimate where most of the work goes in rebuilding a platform like Word or Excel. Getting the UI and common cases working is the easy part. The bulk of the effort is dealing with edge cases and exceptions.

The file format docs don't tell you everything about how Excel or Word actually behaves. And a collection of public files will only get you so far. A lot of the weird stuff shows up in proprietary files that customers have been building and modifying for years.

You get a file from one customer, find a case you've never seen, figure out what Office does, implement it, and make sure you haven't broken something else. Then the next customer sends you another one, and so on.

That cycle takes years. Reading and writing DOCX or XLSX is great, but getting close to full compatibility is a much bigger job.