If I need a bodyguard I'll have to hire someone whose muscles are much tougher than mine. If a server can't handle a DoS/DDoS attack because it consumes it's resources does this mean the firewall has to have better resources than the server it protects? ie bigger RAM, better processor and of course this means the firewall cost more money than the server itself? What is wrong about my assumption?
If I need a bodyguard I'll have to hire someone whose muscles are much tougher than mine
This mean that if you need a bodyguard, you will need someone with very specific capacities allowing him to protect you effectively. He will not need to have the same study background as you, he will not need to be able to administrate servers, he will not need to beat you at chess or whatever.
In other words, the bodyguard does not need to be "better" than you, he needs to be dedicated to his job.
The same apply for a firewall: he will need reliable network cards, low latency bus. But he will not have to handle all the bell and whistles required by a full fledged web server running HTTP daemons, proxies, databases, application servers, and the list could go on endlessly and the longer the list is, the more you will need RAM and CPU. But, when talking about firewall just handling small TCP/UDP packets to decide to let them pass or block them, RAM and CPU power becomes of less importance.
Recently I went on an interesting article dealing on this kind of subject, do not hesitate to check it.