Also, having a specific image in mind isn't a bad thing. Only trans men whose transition goal is "average everyday dude" or "bear" are praised; the rest are ridiculed.
But cis people have goals, too. Did David Bowie come out of the womb looking, dressing, and acting like that? No. He worked towards it. He didn't always do it in the healthiest or most advisable way, but he put a lot of effort into projecting a very specific image that he had in mind.
It's also such a double standard. Women, cis and trans, are not just allowed but expected to put a ton of effort into curating an aesthetic for themselves. They're expected to spend a ton of money on beauty, fashion, and even home decor to fit an aesthetic. And i don't just mean cottagecore. I mean the aesthetic of a regular cute girl who is clean and healthy. The aesthetic of a quirky girl with a personality. The aesthetic of a good, clean, organized partner. The aesthetic of a loving mother who dedicates herself to her kids. The aesthetic of a cool, fashionable girl. Of a nerdy girl. Of a professional in a white collar job.
Men are expected to put in minimal effort. If they're seen doing too much, it's a challenge to their masculinity, and that's absolutely the case for cis men as well as trans men. Men are the ones who are stereotyped as not wiping their asses, they're the ones who don't wear makeup, who can get away with messier homes or a lack of decor without anyone commenting on it, who aren't supposed to know the names of different styles of shirts or shoes. They dress boring. Their hair is short and practical. They don't express themselves too much because that's a girl thing.
So people, subconsciously or not, protect those expectations onto trans men, who have often gone our whole lives being expected to curate an aesthetic and put effort into our appearances... and it turns out, some of us like to do that when we have control over it and it's not forced on us. We want to be men, but we want to be men who look cool and feel good to be. After all, we're going to do much effort for it.
A lot of cis men would do the same if they felt they could get away with it.. Alternative men, who have more leeway because they're alternative, LOVE to curate aesthetics and I've seen straight goth men pour tons of money, time, and energy into protecting elaborate images of who they want to be - much more than most gay men or even most women would because they have a community surrounding them that is affirming towards their presentation and their belonging within that community, whether they're the guy who shows up in a basic band shirt and black jeans or the guy with contacts, makeup, a wig, and an elaborate costume that cost hundreds of dollars.
Trans men have to fight so hard for our masculinity, and we already know we're not going to be taken seriously as men. So a lot of us just say, fuck it, if that's how it's going to be, I guess I'm just gonna work hard to be exactly the kind of man I want to be, because what's the point in transitioning if I'm still constrained by gendered expectations that prevent me from dressing the way I want to?
But the people ridiculing us usually haven't done much self-reflection regarding the way they view men, masculine gender roles, and how patriarchy forces them on men (it does!).
So it's allowed when it's David Bowie because he's cis and no one is going to question his gender. He's an artist. An eccentric. He made a ton of money off the fact that women found his version of male gender presentation sexy.
It's allowed when it's a cis gay man doing ir because, again, no one is questioning his gender, and he's fighting big, evil masculinity, so it's acceptable.
But a trans man is choosing to be a man, so why isn't he conforming to every single expectation of what masculinity is supposed to look like? Why does he want to be feminine? He must not really be a man, because only women and queer cis men wear makeup. He's probably a fujoshi girl who doesn't know what real men are like after a lifetime of mooning over fictional characters because girls are silly and boycrazy and don't know their own minds.
It's not dissimilar to how cis women who aren't hyper feminine (as long as they're still sexy and appear fuckable to the male gaze) get praised for things like wearing shapeless t-shirts (if they have the right bodies) and wearing minimal makeup so it looks like they're wearing none at all, but if a trans woman skips one day of shaving or doesn't tuck, she's treated like a pariah and accused of being a man fetishizing womanhood, shamed for her appearance, and ostracized from women's spaces because she's not trying hard enough to be the quintessential traditionally feminine woman.