What's the point of rootwait\rootdelay?

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http://unix.stackexchange.com – rootwait and rootdelay are used in situations when the filesystem is not immediately available, for example if it's detected asynchroneously or mounted via usb. The thing is, it should be obvious based on the root bootarg if that's the case or not, so why can't the kernel realize automatically that it needs to wait for the filesystem to appear? Are there some technical constraints preventing this automatization from being implemented? (HowTos)