Many links that you see on a page may not have been tested for some time and have gone defunct, or changed their address. Many think of the Internet as one huge library. It is not. A library has a trained staff to catalog and store information. We are dealing here with a fast-growing media and many rough edges.
Some sites use automatic crawlers to check all the links, and report back on their status.
That bogs down the Net and sites with continuous queries. Think of a page with links as an apartment manager where tenants don't notify anyone when they move. For instance, I have a noncommercial web page which I share with friends. Frequently the links change, and since there are many of them, they may not be continuously up to date. I fix the dead links when I find out about them.