So we developed the service to check every link on every page every month. Giovanna Ostacchini, Apple's overworked webmaster in Australia was delighted. LinkAlarm emails me a summary of our problem links every month. We jump straight to our personal online report and fix the links there and then. With so much information, maintaining our site is a piece of cake.
Web hosts are busy people.
To run a check of that size takes 4 hours, a dedicated computer and someone to oversee the process, and that's before even one link has been re-established or the software to run the check has been purchased. And webmasters are human too. They forget to check their site. The first they know about problems is when their customers start telling them their links are down - by that time the damage is done. If you run a website it might be worth visiting LinkAlarm.