Because he also needs FX, and combined it may work out cheaper if he goes processor way.
Although Metallizer or DuHast would hit the distortion spot on for the genre is has mentioned.
Ali Akber dude, you are asking for a strong and heavy distortion. All the processors can perform pretty good in that domain.
Processors start showing their true colors when asked for light or mid distortions, where subtle nuances are called for.
PS: Boss Mt-2 and the likes may not suit. They sound thin and are much better for thrash metal. This requirement calls for a thick distortion.
You donít get what you deserve, you get what you negotiate.