Why is a placebo always necessary?

Why is a placebo (almost) always necessary in science?
Let me explain: Other then in diseases where improvement is based on subjective account, why would a placebo be necessary?
For example, in a study where they are trying to see if a particular drug will help reverse baldness, why would they need a placebo group? There is no way you can possibly make your hair grow back any means other then pharmaceutical interventions, so any increase in hair count would have to be entirely attributed to the drug, no?
Or like an HIV vaccine, there is no way that you can cause yourself to develop anti-HIV antibodies (other then infection), so what is the point of using a placebo?

I know there any many different study designs and some studies don't use placebo, others use active comparators, or both, but in a lot of studies I just see no reason to use a placebo.

The need for a placebo is due to the conscious and unconscious effects on the emotional and mental state of a person who is taking medication which they believe can help their condition.

These effects, which are real, need to be separated from the effects of the medicine itself.

As you seem to be alluding to, placebo effects are much more pronounced when the result of the malady is subjective - e.g, pain or discomfort, but the effects can exist even for something like hair loss or HIV.

As you mention, active comparison studies can serve the same purpose.

So the results can be compared to a control group.

Because some people think that the meds are working and they really arent doing anything. It is also helpful for side effects to see if they are caused by the medications.

it is used as a control. a control is necessary to any valid experiment.

Results need to be measured, Even if the desired affect of the medication will not happen with a placebo. There is still the "side effect" route, the placebo is given to see the mental/emotional effects of the "so-called" medication.
To see if people mental create a result.

But that's exactly the point. A few outliers in the non-treatment group will have increased hair growth, and a few people in a non-vaccine group will test positive for anti-HIV antibodies (whether they actually have them is immaterial). It happens with monotonous regularity.