Part of this is driven by necessity, for example, cellular network chips are typically binary blobs, etc. as mentioned, the heart rate sensor is a binary blob and that's likely because there are no good OSS solutions for those components.

A lot of battery firmwares are closed source, the way that they fixed this for the early pinephone was literally just staring at a memory listing and aiming a heat gun at the battery to see how it reacted when it went hot.