They mostly drink it b/c they feel like they need an instant boost of energy.
I don't think they are going to get that dehydrated from a can of soda. Also, they are constantly hydrating themselves between changeovers.
I remember reading an Sampras interview after his famous match against Corretja at 96 US Open. He said he started drinking some Pepsi b/c he felt like he needed some energy so he wanted to take in some sugar.