Your initial assumption isn't completely accurate. It's not about any life, it's specifically about sentient life, i.e., nearly all animals. It's also not strictly about the life of animals, but an opposition to cruelty and exploitation. Veganism itself as philosophy doesn't explicitly address abortion, but those underlying fundamentals are the same reason why I'm pro choice. Forcing someone to give birth means controlling their body which is the same thing vegans oppose when it comes to exploiting animals. It's also cruel because it can be physically demanding, sometimes painful, and include risks to their health and life. I can appreciate that someone may be morally against abortion, but I don't believe that gives them a right to force those views on another person via the power of the state.