Why do transition metals have similar properties even though they are not in the same group?

All the transition elements have 2 electrons in their outer shell (4th energy level). If one element has one more electron than another, it is because it has one more electron in its inner shell (third energy level). So instead of adding an electron to the outer shell, transition metals with more electrons add it to their 3rd energy level. Therefore the number of electrons in the outer shell stays the same (2) and since this is the case, the properties are similar (for example, for non-transition metal elements such as the alkali metals, all the alkali metals have the same number of electrons in their outer shell (1) and thus they have similar properties. The same is true here.