That disk space is still worth 18 bucks on a ssd between 1/3 and 1/4 of the purchase price of the game new. This isn't an inconsiderable sum.

Also cpu time to decompress audio is microscopic it might actually be faster especially if the consumer has a hard drive to read 1/10 of the data and decompress.

Additionally there is probably a smart middle ground between keeping the data as small as possible and 50GB of raw audio.