But if you're happy to do that, there's no reason it couldn't be offered by a compiler. However it's only useful for an unusual use case, when you have data that could just fit in memory, but doesn't fit in memory without the bit-packing.
All of these are things that you can do manually, and that you can leave to the compiler and hope that it'll pick the correct one, but currently good luck telling the compiler that no, you really actually want to do <x>. Or to try <x> and <y> and see which is better. And the amount of time required to do so manually adds up, even though they are all things that could be done by a compiler.
I personally wish that you could specify / annotate the range that you are actually intending to store in an integer (and have it optionally bound-checked) anyway, but that is another matter.
(Actually, I wish that you could specify types with an arbitrary (probably-pure) boolean function to indicate if something is or isn't a valid value in the type. But that is another matter indeed.)
Many high-level languages require programmers to specify what you want to store ('an integer between -6 and 43, inclusive'), not how ('6 bits'). That typically makes it possible for the compiler to do just that. For example, Pascal has the 'packed' keyword (http://www.gnu-pascal.de/gpc/packed.html)