A few things.

One is you're probably seeing an attractive subset of trans women, if your main exposure is porn or models or actresses. Also, a lot of them start hormone therapy from a young age, which literally makes their bodies more womanly. Also, there's some correlation between body type and gender identity. A higher percentage of feminine-looking males will go trans than masculine-looking males. Also, frankly, we underestimate how hot guys can look as women if they put everything into it - hair, skin, makeup, fashion, etc. Most good-looking dudes can be converted into good-looking women, especially anyone who isn't super beefy or rugged in their handsomeness.