why are aliphatic amines stronger bases than aromatic amines?
The strength of a base is determined by its ability to donate the lone pair of electrons. In case of aromatic amines, the lone pair of electrons on nitrogen is in resonance with the benzene ring. Hence it is not readily available for donation to an electron deficient species. This makes aromatic amines weak bases.
On the other hand, in alkyl amines, there is no such resonance. In fact, the alkyl groups are electron releasing groups which increase the electron density on nitrogen. Thus the electron pair on nitrogen can be donated very easily, because of which they are better electron donors and hence better bases than aromatic amines.