I did not mean that they all have exactly the same power. There of course are differences in between bow and bow and crossbow and crossbow. They come in different powers. However historically they all caped about the same power. And that power is limited by what human muscle can draw. You can have bow or crossbow only as strong as archer pulling it is.
Now it’s true that crossbows, with exception of the lightest ones are not pulled by hand. In basic form crossbow is fixed to the ground with the feet and string is pulled up. Unlike bow, this involves muscles of essentially whole body to get involved. This can be further enhanced by using things like belt hooks, goats-foots, spanning hooks and windlasses. Therefore crossbows have usually much higher “draw weights” (which is measurement of how much power it requires to draw them). This makes many people claim that crossbows were more powerful then bows. The thing is, crossbows have much shorter limbs and much shorter draw lengths -which means that they propel bolts on shorter distance. As a result they’re not as effective in transferring energy as bows are. Therefore when it comes to effective power output, bows and crossbows comes about equal in general.