Because the guys who made Mosiac had no idea what they were doing because everything was so new. At first there was this idea that you'd constantly be going back and forth between pages, similarly to how people used gopher back then. You'd always have your hands on the arrow keys and you'd fly past various pages to find what you wanted. But, of course, terminals didn't really have scrolling, but GUI windows do, so html pages were just written longer and longer and we got used to scrolling down instead of moving between pages.

Recently, Andreeson said that he never expected the forward and back buttons to last this long. He assumed they were temporary measures until something better came along. Nothing ever did it seems.

Worse, I tried MS's Edge browser and found that the spacebar is now a back button. I am completely baffled by this. I am so used to the space bar as a scroll down, I can't think what misappropriation of ideas and user testing must be going on at MS right now to make this change. Its truly bizarre and really killed my faith in MS finally getting browsers right.

Yes, and for what it's worth, I feel Space to page down is another oddball one. I guess it's for the supremely lazy who don't want to have to move their finger to the Page-Down key.