Why isn't the maximum security of triple DES 48 * 2 vs 56 * 2?
A meet-in-the-middle attack basically halves the number of bits of the total key size, rounding up. So 3 and 4 both have 2x security and 5 and 6 block cipher encrypts provide 3x security of the original cipher. I think that this is the main misunderstanding of meet-in-the-middle attacks.

The DES cipher itself is protected against meet-in-the-middle attacks, mainly due to how the key is expanded. Note that the key material is related and that there are many rounds within a single DES block encrypt, which makes it different from how block ciphers can be stacked on top of each other. You can see a question about meet in the middle attacks within block ciphers  here (note who is asking the question).
So it isn't that this isn't an active field of crypt-analysis. Actually, many amateur block ciphers will probably fail against the meet-in-the-middle attacks and are thus rendered insecure (assuming that they specify e.g. 128 bit security for a 128 bit key).