Probably because it’s only four bytes of data, and counting/extracting bits takes more cpu time than one AND operation.
Most CPU’s are optimised to work with whole integers (32/64 bit) rather than individual bits.
If memory was a serious concern you could compress it down to one byte as a ‘number of 1s’ counter at the cost of additional cpu operations, but because 3 extra bytes is such a small amount of data, this memory/time trade off isn’t worth it in most systems.
It’d be useful if you wanted to compress some data logs or something with many subnet masks though.