That much corrosion indicates the bike is being left outside exposed to rain, sun, salt air, etc. It may have also been ridden in wet conditions and not properly dried after. As corrado33 noted the solution is to store your bike inside and to dry it off after it gets wet.

It will be very difficult and expensive to find a bike made entirely of corrosion resistant materials like carbon fiber or titanium. A good quality bike with steel and aluminum components can be a reasonably priced option that will last for decades if cared for.