One word, compression. Either by the streaming company to save on size when streaming.
Or because they used the compressed later release version of the music they stream, even though it’s the same piece.
The uncompressed harder to find versions always sound better.
I would hope that the major streaming services that tout the quality of their music are not compressing the files during the upload process; ie, Qobuz who offers CD quality and hires. Although limited compression may be required and acceptable.
As far as the selection of albums available, it's getting harder to find an original release. Most versions are now over-compressed with poor dynamic range. This is the case for the rock/pop genre, even some jazz such as the ECM remasters.
I can always find a better quality release in my CD collection.