> People cycle because it's faster, not because it's easier

That may be true, but the fact is that cycling is a more energy-efficient form of transport than walking. It takes literally less effort to go the same distance.

There is no way bicycling is not less effort unless it is very hilly or bad roads? Random people have no problem keeping up with marathon runners on bikes.

"Walking is essentially zero effort, so I don't see how cycling can really be easier, "

When your bike is rolling also on flat ground, you have literally zero effort to move your position. With walking it is effort, your muscles must work for every meter you make.

Cycling 2km flat (assume okay bike under 12kg) is easier than walking it. Source: have fibro so I really fell compound effort in every fibre.