Because gaslighting is a huge part of that. So the abuser destabilises you, attacks and corrodes your sense of self, your self worth and self esteem, makes you feel like you are going mad, mires you in self hatred, constantly and unpredictably switches between loving and abusive behaviour specifically to destabilise and confuse you. Some of the time, things are great.
In fact, I would say the controlling begins with a malign version of 'love bombing' -the charm, the recklessness, the massive declarations of love, the largesse, etc. 'Swept off your feet', to begin with, nobody will ever love you like this, it's intense and breathtaking, flattering and exciting.
There is also the isolation - gradual - criticism of friends and family, subtly done, and again with a sprinkling of being extra-adoring/passionate, slowly becoming more toxic and corrosive, so that you end up thinking they are the only person who understands you, loves you, will put up with you, etc.
If there has been clearly unacceptable behaviour, but you didn't leave, then there can sometimes be a horrible feeling of being too ashamed to admit it, which can reinforce the feeling that it's just you and them. Nobody else could understand.
And so you can find yourself living in a sort of greyish murk of uncertainty, depression and misery, so befuddled that although you know something is very wrong, it couldn't possibly be your passionate, so-intense, strong, protective abuser, because they are the person you rely on for almost everything (they've ensured that).
And you can't live without each other.