Implicitly deciding between by-value and by-reference could make it harder to reason about the implications caused by Rust’s memory safety checking. Explicitly putting an & before the parameter lets you know that you can still use the value after passing it to the function (and that it’s not going to be moved):
let x = get_non_copyable_type();
foo(x); // Under this proposal, am I going to be able to
// use `x` after this call? If it’s by reference, then
// yes; otherwise, no.
A type like Vec3 can be used after being passed by value (because it implements Copy), but most of the time, that’s not the case. Also, if it is Copy, then you don’t normally need to pass it by reference anyway (and so in this case, probably wouldn’t).