Most commercially pressed BD disks are still only single layer, meaning a disk capacity of 25GB, not 50GB.
BD is also encoded at higher bitrates with less-lossy compression for the video than what you'll find on a download, and with lossless multi-channel audio at significantly higher sample rates. Add on a few more hours of additional (but more compressed) bonus features and you've got your ~25GB of content.
Of course a longer movie like the Lord of the Rings for example, would require even more space than 25GB and it might require a 50GB dual layer disk.
Either way I am not sure what your point is. Even if they're "wasting" space on the disk, it still has the same cost to produce, and wouldn't have fit on a DL-DVD. Violet laser technology is the next-shortest wavelength after red-orange available from a semiconductor laser without DPSS (which isn't suitable for long-term use in an optical drive), so it's not like there really even a practical option for optical media higher in capacity than DVD, but lower than Blu-Ray.