Why is self diagnosing so hated?

Knowing how hard it is to get an accurate diagnosis, and how often symptoms get dismissed (especially if you're AFAB) why do people hate self diagnosing so much? And I'm not talking those people who just look at one TikTok and say "that's like me! I have ADHD" but people hating on someone who has done months or even years of research, but because they cannot afford a diagnosis or something similar, they self diagnose. Why?