What they do is get people to come into their doctor's office to talk about their condition. Many people either don't talk to their doctor about their conditions or don't even come in to see a doctor at all. People don't talk about conditions for several reasons including refusing to admit something is wrong, feeling embarrassed about it, or not knowing/believing that there are things that can treat their condition effectively.
You're seeing a lot of commercials for skin conditions (psoriasis, eczema) because for a long time there weren't any medications that did a good job of treating them - not very effective (60-75% skin clearance vs. 90%+ clearance now), or less than ideal treatment regimens (weekly injections where now there are medications that only need to be injected once every three months, or previously only an injectable medication where now there are oral tablet medications), or nothing to treat it at all (eczema). Patients were told to only expect so much from their mediocre medication and after taking it for a while they tend to normalize the results/regimen and think they are doing "OK" because that's what they were told to expect. Doctors tend to be the "if it ain't broke, don't fix it" and often don't bring up new medications because they don't want to take the time to explain them or deal with the hassle of trying to get insurance authorization.
The skin condition commercials have that imagery because they show people showing off their skin outside where others can see them, something that they've been embarrassed to do (note that it's always warm and they are wearing bathing suits, tank tops, shorts, etc.). The FDA requires that they mention any side effect that occurs more often than placebo, even if it is rare and not necessarily proven to be caused by that drug (could be sample size error, another medication they are taking, etc). You'd be surprised how many people commit suicide because of embarrassing medical conditions, or just withdraw from socializing. In case you don't remember junior high school and high school, kids can be cruel to other kids who are different. Having a little bit of diarrhea pales in comparison to the effects on their mental health.
(I used to work for a company that makes some of these dermatology medications you see on TV. I did the concept testing with patients for one of those commercials.)