Because it's a vehicle you are actually tracking and not the person, which one could argue a cellphone is more like.
The reason being this, a cell phone unlike a car, is something that is carried on the person and therefore is more likely to go with the person at all times. So, you are using a device to track a person weather they are driving down the road, walking around in a building, or sitting in a field. The cellphone is also something a reasonable person has a higher expectation of privacy on naturally.
Now compare that to a car. A person does not have to use a car to move and by tracking the car, you are not tracking a person at all times. You can go to the store without using a car, walk around in a building without a car, etc. Also, one has less of an expectation of privacy in a car vs a cellphone.
I agrue this point with reasonable facts and observations. When is the last time you went to Walmart and left your phone in a big pile with other phones? Cars have windows and anyone can walk up and look inside. I think, conversely, if I walked up and started looking through your phone you would blow a gasket.
So, the lesser expectation of privacy where a vehicle, not a person is being tracked and the way people treat them puts a hugely different twist on the concept of a seizure/warrant.
With all that being said, I'm big on getting consent or a warrant first if reasonable even if current case law says it's not necessary. For example, I could have all the justification in the world to search a car, but I would still ask for consent first. It goes back to the hierarchy of searches and the challenges that can be brought forth in court.
There are to many factors to compare tracking a car to tracking a phone. The phone can be left at home, stolen, left in the car, have the battery taken out, or be borrowed etc. I would argue tracking a car has less variables. It can be borrowed/stolen and thats about it.
The end goal seems to be the same for both. IMO The process to obtain that info should be the same