This will return all symbols at level -1 that are not heads and not numeric:

vars[expr_] := Union@Cases[expr, s_Symbol /; Not@NumericQ[s], {-1}]

vars[{x >= 0, y >= 0, z >= 0, 2 x >= y, 2 z >= y, 2 x + y <= 4, 2 z + y <= 4}]
(* {x,y,z} *)

Why not heads and not numeric? So vars[Sin[x] + Pi] will give only x but not Sin or Pi.

Be cautious when using this solution. I'm sure it wouldn't be hard to find cases when it returns symbols it shouldn't. It also doesn't handle non-symbol variables such as x[1] in x[1]^2+1, or subscripted ones such as $x_1$. Variables does handle these. But variables only handles polynomials, e.g. Variables[Sin[x]] --> {Sin[x]}.