Would you kindly explain why?
The point is that digital cameras in general and light meters in particular perform exactly as advertised.
Whether one is spot metering or using a metering mode that looks at most of frame all the meter is doing is comparing what it is metering to middle grey (18% grey). If one allows the camera to calculate exposure automatically based on this it is unlikely to consistently get the exposure correct. This is because the light meter and the camera overall has no idea whether the metered area should be middle grey or not, merely whether it is, or not. I.E. the light meter is just a one-trick-pony.
Thus, to get the best results out of a camera, and by extension the light meter, requires human judgement.
If the area metered is really required to be middle grey - all well and good.
If the area metered needs to darker, or lighter, then human judgement is required to notice this and expose accordingly. The camera will not, and cannot, do this by itself.
Sometimes an incident light meter might give one better information about how to expose than TTL metering that relies on light reflected from one's photographic subject. But again, how to expose is as much a creative decision as it is a technical one, so one may choose to under or overexpose compared to the meter's recommendations according to what is desired
Again, if one is shooting raw images according to ETTR principles then all one is trying to do is not to blow the highlights while allowing as much exposure as possible. No camera that I am aware of will do this automatically. Most of the time the camera will tell you that you are overexposing, sometimes by many stops of light. Some newer models can help one out here by dynamically showing areas that are blown at the current exposure settings prior to taking the images via live view. One resets exposure settings as appropriate to just reduce the blown areas to a non-clipping level of exposure. This functionality, termed zebra stripes, has been borrowed from video camera technology and is very useful for those who like to shoot ETTR.
While camera manufacturers do talk up the ability of their cameras to shoot in auto everything mode in reality there are so many exceptions and quid pro quo's that the likelihood of the shooting conditions actually confronting one conforming to their best-case scenario is vanishing small. For many individuals the result may be "good enough". For someone who does know how to expose and is shooting raw then the camera's automatic way of doing things is usually significantly suboptimal.
I sometimes use spot metering to quickly meter the brightest and darkest parts of the potential scene that need detail in them, especially if I am concerned that the dynamic range of the scene might be bigger than the dynamic range of the sensor. This is much less of an issue than it used to be and generally now all that concerns me is how bright the brightest parts of the scene are and to subsequently expose such as not to blow the highlights.
From a couple of your posts it appears that you are aware of the shortcomings of the camera but seem to believe that a camera automatically determining exposure should generally deliver the correct exposure. Nobody that I am aware of who understands how metering systems work and how they instruct the camera believes this however.
Light meters and cameras work according to specific principles but cannot, by themselves, consistently deliver the perfect exposure, howsoever defined.
Tony Jay