Saliva would require a warrant, but they can require a breathalyzer test without a warrant. Breathalyzers need to be regularly calibrated, so it could partly be laziness in that regard. But I'm pretty sure it's mostly because they want to prove you are not safe to drive regardless of your blood alcohol content. The longer the stop, the more opportunity there is to find something against you.
Unfortunately, I can barely keep up with daily life, so aside from me being a mod here and there, I mostly rely on you helpful people for the good stuff.