Because that would be like saying, just because all you got is a binocular with very limited magnification/resolving power, satelites for jupiter does not exist, because you cannot see them through it.
It absolutely is possible to retain a control group when developing a vaccine for a disease with CFR and R0 markedly lower than that of the measles. In fact they even had it, until January.
Just because something is not possible, does not make it any less required.