Presumably learning "spreadsheets" at the level taught in schools is much more useful than learning $brandname software's quirks.
Also, at least when I went to school the software versions used were fairly old (partly because of the budget and partly because that is what the teachers originally developed their course for). Those were different enough (think pre-ribbon) to trip those people up who memorized instead of learning what things mean. I.e. "spreadsheets" skills transfer, "this is what the button looks like in office 95" does not.
Schools are for learning not for training for corporate.
