Because water from the tap (like electricity from the socket) is reliably, spontaneously, easily and in high quality directly available. And because during new construction the effort is manageable (retrofitting, on the other hand, may be associated with considerable pain). The outdoor water tap is a piece of infrastructure that simply belongs to a house for me (like the outdoor socket for electricity).
Of course, the outdoor water tap is objectively just as superfluous as the concept of a single-family home itself. And of course everyone is allowed to build their house without an outdoor water tap here ...