Making Policy

A big mystery to me is why Congress is allowed to hide all manner of new policies in unrelated bills. For example, the anti-abortion provision that House & Senate politicians have attached to a huge & important spending bill. Both sides do it; it's sneaky; it makes it harder to fight these policies if you're against them; it means that politicians later can be accused of voting against important bills when the real reason might have been some of the attached provisions. Why is this allowed? I doubt it makes the workings of Congress more efficient? Why not require that each issue have its own bill?