[amorphid]

One way these companies mitigate DDoS attacks is by being huge. If you have a small house w/ one entrance, there's no great way to manage 1000 people trying to get through the front door. If you have a huge house w/ dozens of entrances, dealing with 1000 people trying to get in the building is much more manageable :)

From https://en.wikipedia.org/wiki/DDoS_mitigation:

One technique is to pass network traffic addressed to a potential target network through high-capacity networks with "traffic scrubbing" filters.

[oneplane]

But if all those people get in and all try to get into the single elevator in the building it will be a problem :p (link DoS vs. application DoS)