Slippery surfaces play a big part, as to heavily scented products that are way stronger to dogs' noses than ours (and I find them pretty strong myself), but I think a lot of it honestly comes down to choice. A dog playing in water chooses when he goes in, how wet he gets, when he can come out. Most people don't take this approach when it comes to bathing. There's a process called cooperative care that teaches a dog to actively engage in handling, including grooming and vet care, by allowing them to have some control over the process. Most dogs (and animals, I've seen this used on anything from tigers to gorillas to cockatoos to otters) become way more relaxed and willing to let you do weird or uncomfortable things to them once they learn they have the power to say no*. Check out Deb Jones if you're interested in learning more, she has a Facebook group and book.
*Obviously people who do cooperative care don't give animals a choice when it comes to urgent medical or grooming problems that can't wait until the dog's comfortable with the process, but often they find that a dog who's allowed choice in most aspects is less stressed and recovers more quickly when they have to go through something they don't have a choice about.