Compression algorithms generally rely on sensing patterns in the data to allow you to store just one example of that data and where it repeats, instead of storing it all fully. This is extremely visible in H264 and H265 for video, where the first is easily 1% the size of the raw video data, and the second is easily 1/10th the size of that, since it can detect more patterns to compress.
White noise means your mp3 is basically the size of the uncompressed data, instead of being 5-25% that size (stat from Wikipedia on compression ratio of mp3). This costs Spotify more for storage and streaming bandwidth.