If gender is psychological, then why is surgery so necessary for trans people?

Gender-affirmation surgery is not necessary for a person to be trans, as you can be trans without having any surgery or procedures, since your gender identity is psychological and not biological. Gender-affirming surgery is a part of what is known as gender expression, which is the physical means of expressing gender externally, whether it be biological, like surgery or hormone treatments, or even non-biological, like the way you style your clothes or hair or whatnot. Your gender expression does not inherently define your gender identity, since your gender expression is merely, as the name…  Read more implies, a form of expression. For most people, denying them their gender expression, whether trans or not, is very emotionally, socially, and/or psychologically damaging (a significant part of the high suicide rate within the trans community, for example), hence why allowing people their gender expression is often "necessary" for their mental well-being.