Reply #50: It was a design limitation at first, for entirely technical reasons.

[boppers]

A 512-byte packet can only hold so much data.

There are only 13 server cluster entries (A-M). If you managed to take out all 13 clusters at the same time, the 'net (on a DNS level) effectively starts falling down. This has been attempted, but not successfully, mostly because of *massive* levels of redundancy and security that are now built in... you would have to maintain a concurrent attack on/through thousands of machines, stealthily, for many hours, without anybody noticing and taking countermeasures.

See:
http://en.wikipedia.org/wiki/Root_nameserver
http://en.wikipedia.org/wiki/Distributed_denial_of_service_attacks_on_root_nameservers