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What is their goal??? These folks must know that they're not going to get in, but it doesn't stop them from trying.

You nailed it. This is a very good description in plain English of what IT professionals call a "brute force attack".

These people have compiled a list of commonly used usernames and passwords. Humans are terrible at making unique choices so they really do tend to use variations of the same commonly used, easy to guess usernames and passwords. That's exactly what the attackers are trying to exploit. They are shooting in the dark knowing that they have a fair chance of hitting a target by an accident. Given that the virtual ammo they're using is inexpensive and practically unlimited of course they'll try to attack any site they get their hands on.

For what it's worth, I have a test site on-line which is completely devoid of content. The only page it has reads something along the lines of "this is an empty site to see which idiots among you are going to try and attack it". Guess what? Dozens of attacks every day with variations on ridiculous usernames and passwords (like admin / 1234567890). Oh, well, they get auto-banned and I get a good laugh.

All this is bothersome, but it can be handled. I just don't understand the reasoning.

Even if they manage to get access to one out of one thousand sites they can install malware on it. This can be used either as part of a botnet (part of the decentralized command and control network or an attack node itself) or as part of a spam campaign. Both of these are very lucrative among criminals. So it's the most common of the three motives for crime (money, the other two being lust and revenge). No surprise here :)