It would not be very hard to ride your bike uphill if you could put it in the lowest gear and go slow.
The problem is that bikes become unstable at low speeds. It will wobble and you’ll fall. So you need to achieve a certain speed to avoid it wobbling, and reaching that speed uphill is hard. You either need a higher gear and lots of force or a lower gear and lots of leg-motion (many rotations of the pedals). That may be more power than you can produce sustained over many minutes.
When you are walking, you can go as slow as you want and not fall over. You can take as long as you want and use less power over more time.
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