Because they aren't meant for everyone to have them right off the bat. If you want one of them so badly then wait a month to get one when they're like 10k or less on the MB. Besides, they aren't that small of a droprate to begin with. You're just not spam running the dungeons to increase the sample size.

Speaking of which, I have the mats for the Atrophied Atomos and that dumb wheelbarrow D: