Hypothyroidism is mostly not preventable, and neither is hypertension for the majority of people who have it- it can be easier to control with lifestyle interventions, but for many, this isn't enough without medications.
Likewise, most people's cholesterol levels hover around a geneticaly determined set-point. Weight loss or weight gain can have an effect, but not always a substantial one. And the whole point of treating high cholesterol is to PREVENT myocardial infarcts and strokes, of which statin medications are remarkably effective.
Diabetes is probably more preventable than high cholesterol and hypertension, but once again, our goal with diabetic patients is to prevent the serious outcomes of the disease, and metformin is a very powerful tool towards that goal.
So yes, there's still a lot of prevention going on with these medications. I daresay most would rather have controlled hypertension rather than all the sequelae of poorly controlled high blood pressure.