Not so, you're forgetting that even experienced cyclists, as they get older, become a little lazier and more unfit, at which point an electric bike becomes and increasingly attractive option.
Up until a few years ago I used to enjoy a 50 mile bike ride most Saturdays, but when I reached my fifties it become less fun and more of an ordeal, so when I discovered electric bikes it gave me a new lease of life. As the OP says you still need to pedal but it's great to have the additional power as an option to get you up the steep hills.
I don't consider myself lazy, but, just before the pandemic I was about to give up cycling, because my dodgy heart was struggling. After the first lockdown ended I bought an e-bike and it is one of the best things I have ever done.
So why are the leading cycling countries flat? Is it just a coincidence? Favourable topography doesn't guarantee high bike usage (see most flat areas of England) but it seems to be a requirement.
Or look in this country at say Cambridge V Sheffield!
I have been cycling in Cambridge for 60 years. The most important reason why lots of people in Cambridge cycle is that it is the quickest and easiest way to make lots of local journeys. Make cycling quicker and easier than getting in the car and people will cycle.
When car ownership increased in the 1960s and 1970s, people in Cambridge still cycled on local journeys, because Cambridge never adapted its road network for cars, apart from the east-west route from Newmarket Road to Madingley Road. The most important geographical feature is not the topography, it is the River Cam. Cars only have 5 bridges to cross the river, 2 of which are not much use, but cyclists now have 8 more bridges to choose from. For many local journeys, cyclists have more direct route options than cars. Furthermore, parking a car in Cambridge has rarely been made easy, apart from the Grafton Centre and the retail parks on Newmarket Road.
Back in the 1960s and 1970s the relatively flat terrain helped cycling, because most people in Cambridge could ride a bike with only one gear, which was forwards. Sturmey Archer three speed gears were a luxury, and very few people had Derailleur gears. But that hasn't applied since the 1980s when Shimano gears took over the mass cycle market.
Various local policies since then have helped cycling to retain its predominant position. In the early days these were more by accident than design, including the proliferation of traffic lights, both at junctions and pedestrian crossings, closing of rat runs and traffic calming measures. Cambridge had low traffic neighbourhoods before they were given a name. Even the cycleway next to the guided busway only came as an afterthought. These all further increased cycling's advantage compared to the car.
The first big scheme for cyclists was probably the Carter Bridge over the railway at the north end of the main station, opened in 1989. Since then there has been lots of deliberate local policies to promote cycling, and cycling continues to be the quickest and easiest way to get around Cambridge. It is increasingly becoming the quickest and easiest way to get between Cambridge and surrounding villages too.
The biggest downside for cycling in Cambridge is probably security, having somewhere safe and secure both at home and at journey destinations. This is of magnified importance for e-bikes because of their higher cost.