Can you tell me about drugs like advair, singulair, (montelukast) viagra. Etc. Why are they advertised. What drugs aren't?

Lawyers. Basically it used to be unlawful and considered bad form to advertise medication to the public. Pharmaceutical companies, advertisers, etc., went to court and lobbied state & federal governments to eliminate these limitations. Any drug can be advertised- but you want to get a return on your advertising dollar, so more likely it will be an expensive drug that has less expensive competitors.