They let the pressure out as soon as the cap is turned enough to lift the seal.

It might also be related to mold release process requirements.

Quote (MintJulep)

They let the pressure out as soon as the cap is turned enough to lift the seal.

I think this is the right answer.

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Chris, CSWP
SolidWorks
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The molds have moveable segments that form the threads. These segments move in to allow the cap to be ejected without having to be unscrewed. So continuous threads are more complicated to manufacture in addition to having other drawbacks such as more stress caused by plastic shrinkage during cooling. To contain the pressure in the bottle, the threads have to be large and coarse. Thicker sections are more difficult to mold due to the greater shrinkage, causing sink marks on the surface, cavitation voids, and stresses that can cause warping or cracks. The interrupted thread acts as a stress relief (circumferentially), and it saves on material.
Fine threads are made continuous because the caps can be popped off of the thread form while the cap is warm and still soft, like rubber.

The gaps between the interrupted threads are not filled in during casting there will be ridges of plastic formed extending into the collapsible mold itself.
There must be mold sections inserted into the gaps between the threads to contain the injected material.
When these thin sections are withdrawn, the mold may then be collapsed.
Those inserts may be either plain or threaded.
Plain inserts may be withdrawn directly.
Threaded inserts would have to be retracted inwards to clear the threads before being withdrawn.

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Ohm's law
Not just a good idea;
It's the LAW!