It makes a lot of sense if you think about it. In classical days, singers read off single sheet voice parts. The tenors just had the tenor part, no indication of what anyone else was doing, the basses just had the bass part, and likely all the basses were looking over each other's shoulders at a single copy by the light of one flickering candle... Think of how difficult it is to sight read if all you have is your part, without being able to relate it to the rest of what's going on. If you play the violin, you put your finger in the right place on the right string and you reliably get a certain note, but if you are a singer, you have to pluck your note out of thin air. It's really hard to do accurately without being able to see the complete structure of the harmony you are a part of unless you happen to have perfect pitch, which is a rare gift. Modern sheet music always gives singers all the choral parts together plus a reduction of any instrumental parts. We're in the age of modern printing. It's cheap and easy to do. Classical period's solution to save time laboriously copying notes onto handwritten sheets was to have the brass double the singers so they couldn't get lost, and to strictly enforce voice leading rules so that parts tended to move by predictable stepwise motion as much as possible, rather than by leaps, which people are more likely to misjudge. It was sheer self-preservation on the point of composers. The alternative was a train wreck every time you premiered a new piece. It's not like singers could go out and listen to a recording of the thing before they tried to sing it either.