Why the files are plain text
The projection sets in this archive are distributed as comma separated text wherever the source allowed it, and the reason is dull enough that it is worth stating plainly: spreadsheets are documents, and these are tables.
A workbook carries formatting, formulas, hidden sheets, cached results and a version of the application that wrote it. Some of that is useful when a human is the reader. All of it is friction when the reader is a script totalling ten thousand rows across a dozen systems. Plain columns load the same way in every language anyone is likely to use, and they stay loadable long after the program that produced them has changed its file format twice.
There is a preservation argument too. The oldest sets in this archive are more than a decade apart from the newest. Plain text survives that gap without ceremony; a binary workbook is a gamble on somebody still shipping an importer.
The cost is real but small. Text files carry no types, so a column of ages and a column of batting averages look identical until something reads them, and leading zeroes in identifiers are easy to lose if a spreadsheet opens the file on the way through. The fix is to treat the file as data and not as a document: open it with the tool doing the analysis, not with the one that wants to reformat it first.