Usually, but on the other hand: Geddy Lee, Sting, Mike Rutherford (more so for Mike + The Mechanics than Genesis, of course), Paul McCartney, Brian Wilson, Roger Waters, Lemmy, and Peter Cetera.
I suspect the relative rarity of bassist frontmen is that it's arguably more difficult to play bass than rhythm guitar while singing.
Okay.
Here's a list John Paul Jones, John Deacon, Cliff Williams, Ronnie Wood, Adam Clayton, Tom Hamilton, Alec Such, Randy Meisner, Tim Schmit,John Illsey, Cliff Burton, Jason Newsted, Duff McKagan, John Entwhistle, Francis Buchholz, Michael Anthony, David Brown, Ross Valory, Krist Novoselic, Rick Savage, Nikki Sixx
All these are bassists from some of the top selling bands of all time. Unless your a fan of these bands, you're not going to have a clue who they are. (Unless of course because of notoriety.)
If someone names these bands, more likely than not, people will be able to tell you the lead singers name and maybe the lead guitarist.
Then there is Sting and Paul McCartney and Geddy Lee, who are bassists AND lead singers.
Other than Paul McCartney with the Beatles, all the people I listed, were with bands that sold more records than the bands you list included.