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.

Depends on the distance run; having crewed an expedition in Death Valley amongst other places I think there's something to say for the complexity that long distance running can bring.

Cycling tends to have more expensive gear, but have you ever heard runners geek out about shoes, socks, chafing remedies, mobile hydration solutions, “gels,” etc.? Definitely not opposite ends of the spectrum.