It’s probably mostly done out of carefulness. I’d do the same if I were to write my own implementation of a file system.

Bugs and mistakes in the code are already not great when implementing the read part. But if you mess up the writing well now you’re gonna cause corruption of existing data. No one likes that :)

Better to stick to just reading for a long while, and weed out most of the bugs and getting an even deeper understanding of the filesystem, before starting the implementation of anything that will modify the data.

I would expect writing to be much harder on all file-systems. There's only one way to read the data, but there could be any number of strategies for deciding where to write new data.

Reading is always easier to implement than writing. Only writing has the potential to actively corrupt the filesystem or leave it in some inconsistent state.

Generally, the more modern the filesystem, the more complicated writing becomes.