I had SRS, and it was great for me, but it's obviously a very personal thing. And it's certainly not cheap, easy, or without the risk of complications. People latch onto it because many people just fundamentally believe that genitals=sex=gender. And genital configuration is the traditional definition of "sex." Some contexts still work this way. In most of the US, prisons operate based on genitalia, not personal identity, secondary sex characteristics, or legal sex. In most of the US, if you were to end up in jail, you would be sent to a men's prison.

It's just a deeply, deeply rooted cultural thing. Genetics and chromosomes weren't even discovered until the mid 20th century, and even chromosomes aren't really all there is to a person's sex. But for centuries prior, your sex was simply your genitals, your gender was your sex, and you genitals were unchangeable. That centuries of cultural inertia is what is being resisted when someone with typically-male genitalia adopts a female gender. Even if you are on HRT and gain female secondary sex characteristics, you still have the primary sex characteristics that were traditionally one-and-the-same with the male gender. It obviously isn't that simple, but for centuries of western history, it was.

Among some, both cis and trans, there's also a sense of needing to prove that a person "really is" trans. Most of the effects of hormones are reversible. Genital surgery isn't. Some people will always be more comfortable accepting someone's gender identity if they "really commit to it." And permanently turning a penis into a vulva is a pretty obvious sign of commitment. Some women just invariably see "penis=threat," and some men see "penis=competitor." There's a lot of cis women who would be more comfortable accepting and treating a trans woman as valid if they know she couldn't force herself on them even if she wanted to. (At least in terms of penetration.)

I know this isn't how things should be. I don't think you need to have bottom surgery to valid in your gender. But genital configuration is something that is burned into the very core of the psyche of how most people interact with each other. It's a fundamental component of standard cis-hetero gender relations. And it's just always going to be a much greater reach for a lot of people to accept a trans person as valid with their original equipment than without their original equipment.

Hell, gender is assigned at birth almost 100% based on genitalia. You were (I assume) assigned male at birth, and unless you were born with ambiguous genitalia, it's extremely unlikely anyone ever tested your karyotype. And for 99% of cis people out there, they've simply stuck with the gender they were assigned at birth, have put zero thought into it, and were simply assigned that gender based on their genitals. The vast majority don't even know their own karyotype; they just assume it. For most people, their gender, their sex, and their genitalia are all indistinguishable. And it's really hard for a lot of people to get past that.