It should be totally possible to run a door-lock controller with minimal power consumption to make it last months on a relatively small battery without constant connection to the grid. You could even harvest energy from the door knob to make it completely independent from external power.
Of course the average "smart" lock is unlikely to be engineered with such considerations in mind and I doubt that the extra effort would pay off financially, but it isn't completely impossible.
The combination locks on individual rack cabinets at a data center I used to visit frequently were powered by the rotation of the front dial. Basically, you twisted the dial back and forth several times to charge it up, then pressed the buttons on the keypad, and a green LED lit and the bar retracted (or else a red LED lit and nothing happened).
This allowed them to (a) not run a power wire to every door; (b) use a long (12 digits) passcode; (c) reprogram passcodes in a few seconds; (d) immediately give feedback on whether you had typed in the right code or not.
IIRC they cost upwards of $500 each. Given that there was already a 24/7 human guard to check you in and out, I'm not sure that they made a smart decision over physical metal keys.
Meanwhile, I have a non-networked, non-wireless electronic combo lock on the front door of my house. It's not hackable without physical contact; if you want to break in, smashing a window is a better route. It's powered by a 9-volt battery and lasts about 4 years per battery.
The big win is never forgetting to carry a key, with a smaller bonus of being able to give friends individually revocable passcodes.
Of course the average "smart" lock is unlikely to be engineered with such considerations in mind and I doubt that the extra effort would pay off financially, but it isn't completely impossible.