It's because that C makes it easy to confuse the two concepts that it's very important to stress when and how they're different.
What creates the confusion in C is that arrays try very hard to decay into pointers at any occasion and on top of that you have "fake" arrays when they're declared as function parameters (something that's probably the most insane "feature" of C IMO. And don't get me started to the a[static n] syntax that solves nothing and introduces even more confusion on top of adding yet an other completely new meaning to the 'static' keyword). Most crucially they behave very differently when it comes to using sizeof.
It's also very important to understand the difference to be able to understand `const char p = "abcd"` vs `char p[] = "abcd"` or `struct s { / ... /; char data; }` vs. `struct s { /* ... */; char data[]; }`.
C really does stress the difference. Look up the definitions of "pointer" and "array" in the C standard. They're entirely distinct concepts.
p[i] is equivalent to (p+i), and that's possible because both the indexing "[]" operator and pointer addition "+" operator require a pointer and an integer as operands. (The pointer needs to point to an element of an array object, but that's not enforced at compile time.) For p[i], it's very common for the pointer operand to be an array expression that "decays" to a pointer.
(3[p] is valid because pointer addition, like integer and floating-point addition, is commutative. It could* have been defined to require a pointer as the left operand and an integer as the right operand. The decision was made when the distinction between integers and pointers wasn't as strong as it is in modern C.)
On the contrary - even in small details, like for example the equivalence of p[i] and * (p+i), so you can legally write 3[p] in C...(==p[3])