Often because the talentless b@st@rds of today cannot come up with anything better.
That said some "covers" are better than the original. "Over the Rainbow" by July Garland was a cute song. The "cover" by Eva Cassidy is sensational.
Rod Stewart (NOT one of my favourites) makes a very good job of many standards in his "Great American Songbook" series ~ for example his duet with Dolly Parton on "Baby it's Cold Outside" is sexy to the point of pornography.
I guess you pays yer money and takes yer choice.