They don't have to be. Cheap department store ones should last a year or two for someone just riding on weekends, it's not a great long term investment but it's good for the cash strapped and many people will give up long before the bike does. It's only a ~2X price jump to something that will last 10 years (more with maintenance) and better suited to someone commuting every day or doing distance rides, way cheaper than a car or public transport.

Sure you can spend thousands on a fancy carbon fiber machine and thousands more on accessories, but it's not a requirement. Even if you want to compete seriously, by the time it makes a noticeable difference manufacturers would be offering you bikes.