They did, but it is misguided. The importance of high guard as defense is a modern mistake. Do you really think that old school guys with their small gloves and bareknuckles hand relied on high guard? NO. With hands like that keeping high guard is a strategic suicide. The high guard only works to a degree because of the big gloves modern boxing have. Take that out and you will see the utter uselessness of high guard as a mean for defense. Of course, that also took away the offensive capabilities of high guard too, sadly.
Except that's only part of the story.
During the era of smaller gloves (and bare-knuckle), shots to the head/face were far rarer. This was due to the possibility of injury to the hands then punching for the head. Compared to today the vast majority of attacks were to the body.
Therefore, because there was a minimal risk of getting punched in the face, they did not need to have their guards as high as they were looking more to protect the body. It's that simple, low risk of getting a punch to the face, less need to protect the face.
With the advent (and introduction) of the "newer" gloves, shots to the head/face increased dramatically because, and this is important, your could punch for that area with a minimal risk of hand injury. As such the "hands-low" guard of old started to fall out of favour towards a higher guards because there was a new risk.
Similar to the way a boxers stance is different to a thai fighters as boxers don't have to worry about being kicked. And a thai fighters stance is different to a MMA fighter as thai fighters don't have to worry about takedowns.
Small/no gloves ==> very few shots to the head ==> minimal need for high guard
Larger gloves ==> increase in shots to the head ==> guard lifted
The fact is you can't be knocked out by a punch to the chin if they can't hit your chin. Now you can talk about movement and positioning all you like but, at some point, you WILL meet someone who has better movement/positioning, you WILL meet someone who is faster, you WILL fatigue and slow down, someone WILL get an unexpected shot in. And, at that point, if your glove is between his fist and your chin, you're not going to get knocked out by a punch to the chin, because the glove will deflect the blow.
However if you're so adamant that a hands-down approach is the way to go, go right ahead with it. But when one of the above happens and your chin gets lit up then have fun watching the stars.
Personally I dislike the VERY high guard (like the peek-a-boo guard) as I feel it has too much impact on your visibility. However I firmly advocate at least having your hands chin-height as an insurance policy.
But then again it's whatever works for you and what you've been taught. If you're happy with a guard below your chin then, for want of a better phrase, knock yourself out. I prefer my gloves at least chin height, it's also the guard that is taught by my coach (who, in his time, was a world-level muay-thai fighter). If you disagree, so be it.
Then again, there's also the variables in style depending on what you have to defend against, hence why boxing, thai, tkd, MMA and so on have different postures and defensive stances.
At the end of they day, find what works for you and stick with it. Clearly nobody is going to change your mind however Id' have thought, by now, that you'de have realised you're not going to change anyone elses mind, no matter how many times you repeat the same things.
It's almost as if, at this stage, you just want the last word, as if that will somehow mean your opinion is right because you flogged the dead horse for longer than anyone else.