Transfer function and weird assumption in circuit
It's a convention. You can get very close to that in practice by feeding Vout into a high impedance amplifier (like a FET).
To model this network feeding another circuit, say, an amp with 10K input impedance, just connect R2=10K across the Vout terminals and model that. The new transfer function will be correct for the new load.
Another common convention is used in radio frequency circuits : impedance matching to 50 ohm circuits is very common, where you can assume the source impedance driving Vin, and the load impedance on Vout, are both precisely 50 ohms (or 75 ohms in television studios, or 600 ohms on telephone lines).
But this is an introductory exercise so you don't need to deal with that complexity yet. Assume infinite load impedance.