1) Because the Supreme Court -- in a narrow 5-4 decision -- ruled them so in the "Betamax case" after the MPAA tried -- and up to that point succeeded -- to have them banned as tools of copyright infringement. The MPAA was sore about this for decades afterward, maintaining that the SCOTUS was simply wrong as a matter of law. My girlfriend's stepfather was a high-powered attorney for the MPAA, and he banned the use of VCRs in the house because to use them to record off the TV was, in his view, unquestionably copyright infringement, SCOTUS ruling or no SCOTUS ruling. And he was one of the foremost experts in the country on this area of the law.

2) The Court, in its Betamax decision, said that "[T]he sale of copying equipment, like the sale of other articles of commerce, does not constitute contributory infringement if the product is widely used for legitimate, unobjectionable purposes. Indeed, it need merely be capable of substantial noninfringing uses." However, the DMCA explicitly closes that loophole. Any tool or device whose primary purpose is to circumvent copyright protection is illegal to use or distribute -- save for certain carve-out exceptions including security research, interoperability research, and law enforcement use -- irrespective of whether it has "substantial noninfringing uses" as would, for example, a tool to strip out Denuvo protection from your legitimately purchased copy of Doom Eternal so you can play the game without Denuvo pegging your CPU. The fact that DRM is an irritant that owners of legitimate copies may wish to be removed is immaterial; providing the means to remove it is still a crime.