What causes some people to not believe in adhd but do believe in depression etc.?

Human experience: Everybody has felt depression at some time and no reasonable person doubts that it can disable someone. By contrast, someone observing the behavior of a kid with adhd might attribute it to poor classroom teaching or poor parenting -- and who knows? My non-specialist take: "why not ask instead whether this person might benefit from a medication trial, and continue if and only if they are helpful?

Ignorance: Ignorance is widely available in the world -- and with it, rigidity to learning from others' experience. If you're asking questions, you're still open enough to be curious -- and that's a good sign of life! there may be prejudice about adhd for all kinds of reasons -- one being that the medications prescribed can easily be abused. The healthiest path is one of keeping eyes open, and learning!