Yes and no, at least for Brussels than I know. Yes, fancy road bike and so on tends to be expensive for second hand. No because, if you look at more "old" bike and you reach to a lot of bike association, often you have kind of repair shops managed by non-profit were you can learn to repair your bike only paying for the pieces your need with someone. Not the best gear will be available but you can easily transfer that knowledge after to maintain your bike on your own.

Brussels and cities in Wallonia can be very hilly and valley-y and require either a good enough bike and/or physical condition. Outside of the cities, Ravel are a good way to travel and avoiding small roads when cars are going way to fast because of habits. An e-bike in those cities is really a nice tools to navigate. The bike infrastructure is not as much there than in Flanders but it will finally come to the same level, I hope. I see way less cargo bike than in the Netherlands too but that will come with the bike infrastructure because riding the city is still tough IMO.

Another angle: or why cars are so incredibly cheap for what they offer. Do keep in mind that car manufacturers make a lot more on the after-sales track than bike manufacturers ever will and some might be willing to reduce the sticker price a bit but raise the price of parts and maintenance to counterbalance that.

I think it's just the difference that car industry is much bigger and much more efficient with its labor. It's also widely different markets so clearly markup is very different. Where automobile markup is often not more than few percent hipster urban ebike market can easily justify 500% markup.