I bought some Thomas English muffins and on the back it says for "Toasted perfection" the muffin should be split apart by hand or with a fork. Why does it not suggest to use a knife?
If you cut it with a knife, you'll tend to get a very smooth surface. If you tear it open, whether by hand or with the aid of a fork, it'll tend to break on more natural places in the crumb, giving a bit rougher surface. When toasted, that tends to result in more contrast of texture - the bits sticking up will brown more and get crunchier.
What Jefromi wrote. Illustrated:
For a muffin with spreads like butter and/or a jelly: the texture of the fork split muffin has nooks and crannys that many people prefer. They provide an uneven spread delivery that can be enjoyable. The higher more toasted peaks also offer texture variance.
For sandwiches (think Egg McMuffin): The fork split may or may not be worth effort. Knife sliced might suffice.
Because the whole point of English Muffins is the create nooks and crannies for the toppings to pool into, giving it a unique flavor and texture. You can't get that from slicing or tearing. Fork-splitting is the original method but now there is an English Muffin splitter that achieves the fork-splitting texture so much quicker, which is great so you can split them quickly and eat them while they're still steaming and so fluffy!