Van Halen songs are mostly . . . hard rock tunes, while Rush are . . . hard rock.
Exactly.
And their lyrical themes are nothing alike. Just compare two songs from the same year (1978): The Trees and Running With the Devil. They have absolutely nothing in common.
Yes, when you intentionally select two songs out of several hundred that are very dissimilar, the result should not be surprising. But how about Red Barchetta and Panama?
Or...let's play a game. See if you can spot the Rush lyrics (no cheating!):
(a) "Yeah, oh yeah. Ooh, said I, I'm coming back to look for you, ooh, said I'm, I'm going by the back door."
(b) "Well I been hustling here, I been hustling there, I been searching for about a week, and I started feeling this strange sensation, my knees are starting getting weak."
(c) "On every wall and place my fearsome name is heard."
(d) "Well, I see you standin' there with your finger in the air, everything we do you want to leave it up to you."
(e) "Everybody's looking for something, something to fill in the holes, we think a lot but don't talk much about it 'til things get out of control."
(f) "Better people, better food, and better beer!"
(g) "I've got a livin' that's rough, a future that's tough, you know what I mean, blankers and boasters, all the bluffers and posers, I'm not into that scene."
All that being said, it's certainly true that the two bands' most significant dissimilarity is lyrics, but you seem to be ignoring a great many similarities to emphasize the obvious differences. Moreover, going back to your original point, why is it surprising that a lot of people like musical artists that aren't complete clones of each other?