The short answer is: Because I want to own a semi-auto for recreational shooting.
I already have good bolt action rifles. One is a Mauser 98 System with a 24" barrel and chambered for 7x64 Brenneke (roughly in the performance class of the 30-06). It can shoot 1 MOA or better. The other is a Steyr Mannlicher chambered for 5.6x50 Magnum (performance is a bit over that of the .223 Remington round). The Steyr Mannlicher shoots 0.5 MOA or better out of the box with bullet weights of 50 grs to 55 grs. There might be a little potential for improvement with handloads.
UTAlumnus already did a good list of advantages of semi-auto rifles. I admit that the recoil of a .223 bolt action rifle is not an issue. However I intend to use it for falling plate matches. With a bolt action rifle you are seriously outgunned by the semi-auto shooters. Changing a magazine hurts as time counts. The .223 is the minimum caliber for those matches.