Why are there some music artists who have many #1 single but never had a #1 album?

..like Rihanna. She has plenty of #1 songs in the billboard but not
one of any of her albums topped the billboard album chart whereas
Taylor Swift who, belive it or not, never had a #1 song in the
billboard but she already have 2 #1 albums.

I don't get it.

and please don't answer "because Rihanna sucks" or " none of them
deserve it.." and something like that..i need real answers
here..thanks you. :)