Concrete Audio Speakers...Speaker Enclosures made out of Concrete ?..

Because they are not thick enough and they are too resonant. You want something that doesn't ring when you tap it. 

I am not talking about these speakers specifically but rather concrete enclosures in general. Are you suggesting that any thickness of concrete will ring? Hard for me to think of a better, relatively inexpensive material to use in a speaker. I will await your solution and response. 

If it was so good most companies would do it. Those companies that have done it haven’t been successful.

A concrete wall could be used but not a box.

Kenjit

Read what they say about these Betonart Arrivato Concrete speakers
https://translate.google.com/translate?hl=en&sl=de&u=https://www.lowbeats.de/test-betonart-arrivato-standbox-mit-perfektem-gehaeuse/

Match these speakers with the $20K Audio Note 300B Meishu Tonmeister amp and your be in Audio Heaven.......

hdcls
Regarding horns: the room matters. I have had Altec 604Cs for ... 43 years now. They DID sound a little horny (tsk tsk) at first (and I have stayed with these speakers because at my price point, to my ears there is nothing that comes close to their performance), but as the other components improved the condition improved.

I am now in a semi-anechoic room, and the condition has largely disappeared.