I like Weezer, not so much Rage, but I'm afraid your numbers are a bit off. In U.S. sales (international was not listed at Wikipedia) Weezer has indeed managed a trio of platinum albums, but they are barely platinum albums -- they have sold a total of 7.7million albums in the last 14 years, on 6 albums, with nearly half of that (3.1mil) off their 1994 debut, which I do own. That is not bad. That is also not remotely on the same planet as a borderline superstar band at the level of Green Day or whoever.
Rage also produced a trio of platinum albums in the 90s, and generally a little stronger in sales than Weezers trio, although Weezer's debut sold more than any of them. Rage racked up 8.5milion albums sold in the U.S during their 10-12 years together, on 4 albums. Again not bad. Both were major bands.
Now for comparison, and I obviously did not raise Green Day as the comparison, but they will do -- Green Day in pretty much the same period of time has sold 22 million albums in the U.S. and 65 million around the world. Pearl Jam in the same period has sold 30 million in the U.S. and 60 million worldwide. For the Chili Peppers it is about 19mil U.S. and 50 milion worldwide (in a slightly longer period than the others). Smashing Pumpkins, who you raised, had about 18million in U.S. sales.
In any case the point would be that Rage and Weezer were/are clearly 2nd or even 3rd tier bands in terms of popularity, or at least popularity that anybody chose to express by buying their albums. Green Day, to take your comparison again, had a single album which sold more copies than the entire discography of either band. So did Pearl Jam. So did Shania Twain. Twice.
We just happen to appear somewhat overrepresented by Weezer/Rage fans on this board, or at least in these drafts.