Plus, taking the data out of [proprietary software app my client's data is in] in csv is usually easy. Taking the data out in Apache Parquet is...usually impossible, but if it is possible at all you'll need to write the code for it.
Loading the data into [proprietary software app my client wants data put into] using a csv is usually already a feature it has. If it doesn't, I can manipulate csv to put it into their import format with any language's basic tools.
And if it doesn't work, I can look at the csv myself, because it's human readable, to see what the problem is.
90% of real world coding is taking data from a source you don't control, and somehow getting it to a destination you don't control, possibly doing things with it along the way. Your choices are usually csv, xlsx, json, or [shudder] xml. Looking at the pros and cons of those is a reasonable discussion to have.