Because "model card" is a set phrase, it's a concept. It originates from a time when they were shorter. Like datasheets, even if it's not literally a sheet.
They could say "tech report" but model card makes it clear that it's a specific kind of tech report.
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I think 'model card' should be a 1 page summary of key info. The report format should be something like a 'model data sheet' (like safety data sheets that you get with chemicals). 98% of people would only want to know the key info, not read a whole report.