Because DMCA makes itself easy to be abused. Off the top of my head, I can remember Microsoft asking Google to take down results to OpenOffice[1]. The first Google result for "DMCA Abuse" reveals this:[2]

[1] http://www.theregister.co.uk/2013/08/16/microsoft_dmca_taked...

[2] https://www.eff.org/press/releases/fifteen-years-dmca-abuse

EDIT: They aren't "inherently" abuses. They are easily used as such.