Because the bar for finding a partner as a woman is "don't be fat".
Anyway, take it from an old man: if you try to change how you act or present yourself for the sake of finding a partner, you're setting yourself up for failure. It's just lying, and there are two outcomes: you get a partner who likes the lie you're telling, and then over time you drop the act, your relationship fails because you aren't actually the type of person your partner wants to be with, and you're back at square one, only older and having wasted both your time and theirs; or, you never drop the act, you lie for the rest of your life, and live a really shitty stressful hollow narrowed existence, having cheated yourself out of being who you are for fear of loss or loneliness.
If people are rejecting you, it means you're meeting the wrong people. The solution is to meet more people. The solution is not to pretend to be someone else -- because eventually you're going to meet someone who would've liked the real you, but they won't know you're you, because you'll be pretending to be someone else. Be true to yourself. Focus on your own goals and values -- like not being fat -- and you'll naturally attract likeminded people. You're better off meeting someone who actually likes you, even if it takes years, versus tricking someone into thinking they like you today.
This is something your father is supposed to teach you but I assume yours didn't stick around, please don't imprint on me like a duckling or whatever.