You’re thinking at a different level of abstraction to the Zig developers. Summing a primitive type is going to be turned into a couple of machine opcodes (but no branch instruction) and will usually be constant time, whereas an explicit function call will require pushing pc to the stack and jumping elsewhere, executing any number of instructions once there, taking an arbitrarily long time, making it more difficult to reason about.
Because on your typical arm mcu, x/y is a function call to a definitively non-constant time function.
And lets not forget soft-fp. Every single floating point op is a function call...
With most languages you need significantly more context than you do in Zig - in C you need to know what preprocessor shenanigans might be going on; in C++ pretty much anything could be happening (operator overloads, virtual functions, constructors, destructors, who knows what else). With Rust you need to know what traits are imported, and if proc macros are involved then anything goes.