Why do most beginners play this line against the Caro-Kann? 1. e4 c6 2. Nf3 d5 3. exd5 cxd5 4. d4 and 5. Nc3?
[FEN ""] 1. e4 c6 2. Nf3 d5 3. exd5 cxd5 4. d4 AnyMove 5. Nc3
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It's not the best way to play against the Caro-Kann, but it's good enough for beginners. It follows basic opening principles: develop your pieces, fight for the center, knights before bishops, though it does violate the rule about moving a piece (or pawn?) more than once. It's true that the Knight on c3 blocks the natural c4 advance, but beginners can't be expected to know that (see below).
If you're seeing this a lot from opponents, don't worry. It will go away (but not completely) as your rating improves and you start playing stronger players. Beginners are commonly told not to study openings, which they interpret to mean don't even learn the first 3 or 4 moves.