At least in the US, many, maybe most, native English speakers spell "a lot" as one word until taught otherwise. Why is this such a common phenomenon?
There are several pieces of (non-written based) evidence that it should be analyzed as two words, including most significantly that it can be interrupted by "whole," or, less commonly, "great, (n) awful." The same phenomenon doesn't seem to happen for parallel constructions, e.g. nobody seems to analyze "abunch, afew" as single words.