Isn't everyone demisexual?
No. This question misunderstands the fact that orientation is about an internal feeling, and not about behaviour. Demisexuals are not defined by the fact that they may only have sex with people they know well, they are defined by only finding such people attractive in the first place. Demisexuals do not ever think that a stranger is 'sexy' or 'hot', which is significantly different to the experience of most people. And this is not to mention the fact that some demisexuals may require an extremely strong / specific bond to be able to feel attraction, such that they essentially never do.
Asexuality and demisexuality are alike in that, the vast majority of the time, neither experiences sexual attraction to other people. The two orientations have a lot of experiences and issues in common, to the point that it’s not unusual for someone to initially identify as one and later realise they’re the other. When it comes to people that we don’t know well, demisexual and asexual people act and feel pretty much the same way - no sexual attraction is present at all.