Because a cashier's check is for a fixed amount; you can't change its value on the spot without going through the whole process again.

Suppose you show up to buy a truck with #30K in cash (the truck is listed for $30K). You inspect the truck, and find that the A/C needs to be fixed, which would cost you, say $3K. So you decide to split the repair cost with the seller, and now the truck will cost you $28.5K. If you have cash, this is simple: you just hand over $28.5K. But if you have a cashier's check?

You walk. In that case, I would walk regardless because I would ask if there were any issues with the vehicle and if a problem with the A/C wasn't disclosed, what else wasn't disclosed?

When I show up to buy a vehicle, the only thing I'm doing is taking it to a mechanic for a look over, and completing the purchase if it's all clear. That's it. The time for disclosure and negotiation is over. Not negotiating things in person (as opposed to over email where I have it in writing) has saved me countless hours over the years and made buying things much more pleasant.