Why is political science not a science?

Most sciences, especially the hard sciences, pride themselves on generalisability of laws inferred from experimental conditions. In physics and chemistry, for example, great emphasis is placed on 'discovering' properties of the universe which operate directly via physical objects and have no major exceptions. Many people who are not knowledgeable in the philosophy of science assume that all meaningful scientific work involves experimentation and observation and so, to them, the Social Sciences often seem questionable because they invoke abstract ideas (e.g.) preferences, markets, the psyche) that are not easily observable and lack controllable initial conditions (e.g.) you can't simulate an entire political system easily). Some detractors, especially positivists, contend this invalidates the ability of politics to be studied scientifically. However, all sciences use abstractions (e.g.) physics uses gravity) in theoretical explanations and all experiments involve the isolation of the experiment from the real world. Laws can be generalised for subsystems, and the major issue of political science, in my opinion, is that politics is a very complex system and so generalisations are difficult to make, especially given that peoples' behaviours are variable. I would argue that politics is studied more like evolutionary Biology, where seemingly objective facts about specifics under considered under a theoretical framework to isolate their behaviours.