Also, studio executives likely think there's not much profit in it, given that the shorts are technically public domain.
That problem was "solved" in some of Paramount's recent transfers by creating a "Special Edition" release (with all the quality that implies).
Namely, they replaced the ending Paramount logo of all the cartoons with the one found in the very first short--including the audio track, which in the later shorts quite often continued on from the sound and music in the cartoon proper. Result: incomplete audio.
(For instance, the ending music in "Jungle Drums"-- the vintage WWII song "Praise the Lord and Pass the Ammunition" -- loses the final bit of its chorus this way.)