Imperative languages and FP are equally arbitrary; the lambda calculus, however, has been studied longer, is simpler, and was designed by Church to be more familiar to mathematicians (they didn't know anything else) in the sense that every term is equal to some value. Those foundations are not mathematical truth, but mathematical habit. On the other hand, the lambda calculus expressed what people though of computation in the 1930s; it was clearly not designed to express more modern usages, such as interaction and concurrency. Indeed, when expressing those, it becomes essentially equivalent to the classical imperative style, as it can offer no better way, unlike more modern formalisms developed for a more modern view of computation.