Because phrases have cultural meanings rather than just literal meanings. This applies to every language and culture and is a big reason why translations can be difficult to understand. White power is culturally tied to the dominant power desiring the oppression or eradication of the minorities, where as black power is culturally tied to overcoming oppression and surviving.
That’s like removing all culture. Every culture will have phrases that are negative and even if we started completely fresh today, we would develop more on our own basically immediately. All words and phrases have their contexts and cultural meanings and that’s not going to change.
You can be proud of your heritage if your heritage isn’t horrible 💀 like I’m white and have talked about how much I love my heritage and am proud of it. My ancestors were actually great people, moved their whole family and left their religion just to fight in the civil war against the confederates (their parents had previously been indentured servants and freed so they were very anti slavery). And a million other amazing stories from my direct lineage.
Saying white power means nothing similar because white people as a majority committed heinous crimes and as such a large majority in the USA, saying white power doesn’t say anything about your ancestry or ethnicity. Their are so many incredibly different kinds of white so if you want to be proud of your ancestry, saying “I love being white” is NOT the way to describe that. Saying “I’m proud to be Italian/Scandinavian/Scottish/Irish/English/Brittish/etc.” is absolutely acceptable.
There is not reason to be proud of being white. We didn’t all experience something together or fight something together that’s relevant to our skin color. We didn’t all come from the same culture or group, speak the same language, fight the same wars, etc. if you are proud of your ancestors or family’s accomplishments or heritage or perseverance through something then say what that is! No one is stopping you from learning and loving your history.