OpenEuphoria

Firstly, the {i,j,k} = <expr> syntax has already been implemented¹.

¹ Bear in mind I can only be authoritative about Phix, however I think it is also true for OpenEuphoria.

Given the way Eu is based on sequences, it somehow "fits". Otherwise, no real reason, except perhaps that

    i,j,k = {1,2,3} 

starts to get misleading

which is genuinely misleading. That absolutely has to return {1,2,3}, otherwise you've suddenly got issues with

if ... then 
    return 1,2 
else 
    return 1,2,3 

and, given there is only one eax/rax, subsequent implications for the system stack, call_back(), etc.