Okay so I'm actually going to answer the "why are they this way" question for those who want to know.
If you are a cistereotypical man, you have no breasts. Your shirt can slide up and down your torso easily without catching on two round bits in front.
If you are a cistereotypical woman, not so much. At least, not if you want your shirt to fit and show you have a waist instead of just hanging off your boobs like a tent. Which is a fashion look - fair - and you'll note that if you go for those shirts you probably also can find them with loose sleeves. Torrid sells a lot of this style because it is a plus size company and it's a flattering style on anyone who has more than one curve to their torso.
[https://www.torrid.com/product/challis-dolman-button-up-tunic-shirt/45142741-17621.html]
However, back to the fitted problem, if you have a shirt that has been tailored to show your waist, then if the shirt gets pulled up around your shoulders and breasts, it will get sort of stuck on the boobs and not fall back down easy.
[https://www.prettylittlething.us/product/seam-detail-fitted-shirt_plt02835?colour=white]
Do you see how the seams tighten under her bust to show off her waist, and then flow back out to fit her hips? That tight bit will get stuck on your boobs and your shirt gets all rucked up and uncomfortable.
So, what does that have to do with sleeves?
A lot, actually. Go put on an oversized t-shirt with big sleeves and lift your arms. You'll notice that the sleeves are connected to a spot that *would* be your under-arm if they were tight. But since they're loose, the sleeves actually connect somewhere by your bust or even underbust. And since you just lifted your arms, you're pulling on that side seam, which means the shirt pulls up from there. No big deal in a big t-shirt, it can fall right back down.
Now borrow a friend and put on a fitted shirt, and lift your arms. You'll note the tight sleeve means the under-arm bit attaches way higher up on your torso. That means it isn't pulling at the side seam of your shirt. Now ask the friend to pinch the side seam where the other sleeve connected, and pull it up to your armpit.
You will immediately notice the problem. Your chest will be compressed, the shirt will get "stuck" and you will have to manually tug it back down, and if you planned on tucking it in, you may also have to tuck it back in.
If you want loose sleeves, buy loose shirts.
If you buy something with a shape for boobs, you will probably find tighter sleeves.
If you buy a size up, you can get a tailor to take in the waist for you while having loose sleeves, but be aware that it may lead to the aforementioned bunching issues if you lift your arms above shoulder height.
Also, you can get a looser shirt like the Torrid one, and pin/belt the waist for a no-tailor solution. You'll have to tug it down every time you lift your arms, though, if you use a belt.