Two reasons.
1) All the steel parts of both cars are typically connected to the negative of their batteries so if you are not careful you cause a short if you let a positive connected jumper cable to hit the anything metal on the car.
2) When you make the last connection there often will be a small spark produced. Ideally you don’t want to make that connection at the battery as there is a very small chance you could cause a battery explosion from the hydrogen released by charging batteries.
I have experienced a lead acid battery explosion from a different cause and I do not recommend. I was fine, a coworker got battery acid all over him and some shards of plastic stuck slightly into his skin.
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