Air is 78% nitrogen, 21% oxygen, 0.9% argon and then other gases and vapors make up the other 0.1%. If you take the same number of molecules of any gas and give it the same pressure and temperature, they will take up the same volume. So, one mole of Hydrogen gas has a mass of about 2 grams. One mole of Helium has a mass of about 4 grams. One mole of Nitrogen has a mass of 28 grams. One mole of Oxygen has a mass of 32 grams. If 2 objects take up the same amount of space, then the object with the greater mass will settle lower than the other object. Helium isn't floating up as much as it is pushed up by the heavier gases that settle as close to the earth's center of mass as they can get. Because Helium doesn't bond with other elements, there's no real force to keep it bound close to the planet's surface. It just keeps getting pushed up higher and higher until it reaches a point in the upper atmosphere where it just kind of expands into the vast emptiness that is space, never to return.
So think about how precious of a resource it is, and how necessary it is for medical and industrial applications. And then question why the average person can purchase it so readily and cheaply at a party supply store, given that we can't really make any more of the stuff (not until we have fusion reactors up and running, that is), so what little we have, we should be conserving. And then ask yourself why there's no market for Helium futures, despite the fact that we know that it will only become more and more precious as time moves forward. You can buy futures in precious metals, but not in helium?
There's a whole lot of stuff to unpack once you start asking questions.