I guess because there's no simple infix operator for every operation implemented by the machine. Also, the addition may be floating-point or 32, 16, or 8 bit, signed or unsigned, possibly vectorized. An opcode is a good place to express these things.

Opcodes are fine, no one is saying everything has to be expressed as an operator. I'd like to see more of a C/C++ intrinsics syntax, but with total control over the order of instructions and register allocation. Basically same power as assembly, syntax close to intrinsics, but you tell it which registers to use, and there's one instruction per line, and they're executed exactly as you write them, with no creativity on the part of the compiler.

+1. Make it look more like a severely restricted version of C.