Why do you believe that so many people are against abortions?
Because we've been taught a number of societal values... namely that people with Uteri should not be allowed control, and part of trying to cling to control over people with uteri is telling them they should not be allowed their most basic human right or that this right can somehow 'disappear' the moment we become pregnant (it doesn't).
We've also been taught a stigma against abortion's over the past century and a half (Yep, that's actually all we've had to fight for our right to this over, before then the church actually provided abortions).
We get shown emotionally manipulative propaganda (And propaganda is an extremely effective way of getting people to believe something mindlessly. Check out the programme 'Love, Hate and Propaganda' which covers the propaganda films shown to the people of Germany during the Nazi regime. A lot of what they were shown was extremely convincing stuff).
We get people using emotionally manipulative language like 'Babies' which stir's a natural instinct of protection and makes us imagine fully autonomous infant's being killed and 'Murder' which is something everyone knows is wrong.
We get people ascribing person hood to fetuses where none exists.
Then we have the mass of people who honestly believe they're doing something good (They actually aren't). They're the people who have bought into the propaganda. They are also unable to separate their hyper-awareness, a unique thing to born human beings and recognise that they were not always as hyper aware as they are now.
They are not willing to separate the fact that though they are people now, and aware and autonomous now, from the very real fact that they weren't always.
They are also unwilling to imagine a world where they simply never existed while forgetting that this is not part of the equation. They say 'I'd be sad/angry/upset if I never existed, so how can you be so mean/care so little about yourself?' without realising the pro choice people aren't saying they 'don't deserve to exist' or that they hate themselves... we're pointing out that they actually would not be upset if they never existed because they would have never existed and been incapable of being upset.