Why get a warrant when you can use a statute instead?
We the People understand and appreciate that law enforcement's job is to investigate crime by collecting evidence to be used in a criminal prosecution. In the DWI context, an officer's primary goal is to generate evidence of a suspect driver's blood alcohol concentration.
Are you aware that there is no legal requirement that law enforcement pursue license revocation when an officer suspects a driver of DWI? Chemical testing may be required at the discretion of the officer. The implied consent advisory only comes into play if the officer chooses to pursue a revocation.
In other words, an officer doesn't have to read the implied consent advisory to a DWI arrestee; the officer has a choice. She has two options; both accomplish her primary goal of gathering criminal evidence, but only one is constitutional: (1) read the implied consent advisory that makes it a crime to refuse a chemical test, or (2) submit probable cause to a judge to get a search warrant. Convenience or Constitution?
From the officer's perspective, of course it is more convenient to rely on a statute to execute a search than it is to go to the trouble of getting a warrant.
But We the People didn't write the Fourth Amendment from the officer's perspective, for government convenience. The Constitution didn't create rights. We the People have individual rights, and We the People wrote the Constitution to ensure that government honors and protects those rights. (And We certainly didn't empower the legislature to create offense-specific exceptions to the warrant requirement.)
We wrote the Constitution to regulate our government. It sure would be nice if government played by the rules.