- Repairability by mere mortals. If you are competent enough to use Excel and are given bad data, you are competent enough to fix it. (Whether the dataset is too big for mere mortals to find the problem is a different issue.)
- Trivial serialization. It is super-easy to dump things to CSV. (This is probably also why there are so many annoying variants.)
- Usable by other tools. You don't need special libraries to read it, so pipelines involving the usual suspects is possible. Importantly, there are a ton of different ways to do this sort of thing, so non-experts can frequently find something that works for them, even if it looks wonky to programmers.
All that said, I hate dealing with them, too.