A simple answer is because its hard. A slightly better answer I think, is that the current crop of more expressive languages (such as not Fortran and including functional languages) are harder to analyse and even harder (much more so) to synthesize in a sufficiently coarse grained manner (parallelized but now less efficient due to overhead say).

The key is a way of providing non-obtrusive (to the programmer) hints to the compiler - whether by annotation or the type system or whatever.