Dealing with the Hillbilly Attack – Why not ...e5?
Gives yourself an isolated pawn, strength of most caro kann setups is that pawn chain spanning from h6 to d5. This In conjunction with the open c file. Pawns can’t move backwards so you lose the ability to play e6 and keep that pawn structure. E5 isn’t *bad* but will lead to a more open game and with that sniper bishop bearing down on your f pawn it could be hard to hold those pawns.
Engine says it’s because white has d4 to challenge them.
Yes, the opponent will challenge the center, but that's chess, isn't it? Black can defend with the minor pieces, just like White does in standard 1.e4 e5 openings when Black strikes back. Personally, I would prefer to have that massive space advantage and defend it, rather than not having one.
trw0311 thank you, great answer.
As Black I used to play the Caro-Kann a lot and sometimes got good results, though more recently as Black I favour e4-e5 openings. If White played the Hillbilly, more often than not I won. Wikipedia's article on the C-K maintains that the Hillbilly tends to be played by people unfamiliar with the C-K.