Space junk is hypersonic (LEO orbital velocity is ~7.5km/s ie ~mach 22) and thus burns up when it hits the atmosphere - and if it's large enough to not all burn up before it sheds most of its speed, it'll plummet to the ground rather than floating around in the air until taken down.

If you're thinking hydrogen fuel tanks, their walls are much thicker than weather balloons and would drop like a stone if one somehow survived reentry (eg protected by some larger structure that burned up).