a. because poor people exist.
b. because people with abusive parents who won’t allow them to get a diagnosis exist
c. because people without insurance exist
d. because people who live in areas with really shitty medical care exist
e. because people who have faced medical abuse exist
f. because people who have severe phobias of hospitals exist
g. because everyone deserves to be able to know what they have going on in their brain and be able to understand it and have a community and make a little bit of sense of all the stuff swirling around in their head regardless of all the above points.
Bonus: why I think self diagnosis is valid:
because the literal list of symptoms for mental illnesses is, in this day and age, available on the world wide web for all to see (the DSM V). You can look at it and see the same exact thing a doctor/psychiatrist would use to diagnose you.