Why there is no memory card with 3, 5, 6, 7, 9 GB capacity?

Addresing of memory cells is based on binary numerical system.
When converting to decimal, the biggest possible values for shortest amount of address bits is 128 MB, 256 MB, 512 MB, 1GB (=1024MB), 2GB, 4GB and so on - all of them are powers of 2.