cus it ties things together and has the added bonus of making things the slightest bit easier
if every song in the soundtrack were completely unique (or most of them were), the game would probably feel a lot less cohesive (and there would probably also be fewer songs). the fact that hopes and dreams has the lietmotifs of undertale and your best friend is incredibly appropriate and awesome. i certainly don't think it detracts from the quality of the music. it's a choice of musical design that's best understood in the context of what it provides to the game itself. in an ordinary soundtrack it'd probably be weird