Depends on the DVD. Some of them do look terrible, but some aren't too bad. Probably depends on how it was transferred and mastered and what bitrate they used on the disc.

Remember those DVD releases that had plastic edges and cardboard fronts/backs? That's an era for you. They're always max 480p, sometimes even 480i, and often single layer, dual sided. Those came out when the Sony was still making Trinitron CRTs that could barely do 720p.