One reason which matters with libraries is that slapping constexpr on a function is, in a way, a promise by the author that the function is indeed meant to be constexpr.

If the author then changes the internals of the function so that it can no longer be constexpr, then they've broken that promise. If constexpr was implicit, then a client could come to depend on it being constexpr and then a change to the internals could break the client code.

Disclaimer: this is not a normative statement.