For practical computing purposes, every algorithm is O(1)?
Sure. Time and memory are both finite, thus $O(1)$. But that isn't a very interesting (useful) result. Complexity is studied precisely because we do care how much time (or memory, or whatever other measure is of interest) algorithms take as a function of the input (size), even if we aren't willing to wait "finite, but very large" time nor to equip the machine with "finite, but very large" memory. We'd want to have an idea of how long we have to wait, or how much memory is enough.