When I've explained to people who've "theyed" me that I use he/him pronouns exclusively and not he/they, I usually get treated like I've done something wrong. For the nth fucking time: I have nothing against they/them pronouns for people who choose to apply those pronouns to themselves.
However, here's why I don't want anything other than he/him used to refer to me:
The first year that I was out as trans, I did, actually, give people the option to call me they/them or ze/zir in addition to he/him, as a courtesy to my friends getting used to the idea of me as not-female. What ended up happening was that literally no one used he/him for me and everyone used the gender-neutral pronouns, which - whether this was true or not - my brain translated this as "people don't see me as male enough to use male pronouns for me" and that's dysphoric ickfeels. So I stopped using they/them altogether, and that was back in 2014 and since then I state he/him publicly everywhere I'm online; you have no excuse to not know what my pronouns are, especially if you've known me longer than a month. [Before this strawman comes up, no obviously I don't expect someone like a cashier at a grocery store to automatically know what my pronouns are. I'm just talking about online interactions.]
Also, over the last decade since I came out as trans and began living as male [2013-2014] I've noticed that people who know someone's pronouns are he/him or she/her but think someone isn't masc enough to ID as transmasc or isn't femme enough to ID as transfem will use "they/them" as a way of subtly and deliberately misgendering the person without outright calling the trans guy "she/her" or the trans gal "he/him" and getting dragged for intentional misgendering. And when I say "I've noticed", I mean that "this has directly happened to me as well as several other trans people I know".
Hope that helps.