Many smartphone cameras lack the ability to turn off burst shot mode.

The burst shot feature on smartphone camera software is almost always not helpful, only annoying. All it does is spam the storage with useless near-duplicate photos.

"Then simply don't hold the camera shutter button!"

Sometimes, this happens by accident. Or the phone has an I/O lag in the moment of releasing the shutter button, so the release of the shutter button is not registered and burst mode is initiated after the I/O lag.

The only purpose of burst shot seems to be making many low light photos to find one that is not shaken. Even then, there must be an option to turn it off.

Also, the point-and-shoot intuition of holding the camera shutter button to set focus and exposure, and releasing to capture a photo is far more convenient. On newer phones, that has been replaced with highly annoying burst shots.

"Then use a third-party app that does allow turning off burst mode."

The problem with third-party applications is that they are awfully slow, since they can not be optimized for a specific device like pre-installed camera applications are. This slowness, as one might expect, leads to missed moments.
On some smartphones, third-party applications can not even access all camera features, such as 2160p video recording. Some phones use a proprietary API that can only be accessed with the pre-installed camera app.