Yes, generally speaking it is either because bots can use significant cpu power or because running an IRC server is like waving a big flag *and* walking around with a bullseye painted on your back.
I've helped out administrating a machine which runs an IRC service, and it usually got attacked at least once a day. Most attacks were small (~100 packets per second for 10 minutes) but at least once a week there would be a flood on the order of 1000 packets per second for half an hour or more. FreeBSD never had any trouble discarding all the packets, but it still clogged up the network and made the connection slow for everything else.