The optifine feature greatly lowers the FoV, a zoom shader would magnify what's onscreen. Technically those produce similiar results but thing is, the image that ReShade gets from the game is fixed resolution - means that a zoomed image would be highly pixelated because you essentially blow up some area with like 200x100 pixels up to screen size. What you get is some highly pixelated and blurry thing, not a sharp zoom like in Optifine. Plus the mouse reacts the same way with and without zoom (on ReShade that is), so all movement onscreen is amplified with magnification. Move the mouse a tiny bit and what you focused at is gone.
I know why you want this, having a scope on every weapon in any shooter like say CSGO would be damn useful, but trust me, I tried back in the old days, attempted using SweetFX as scope for shotguns on BF3, it's not really useful.