Kobo in my experience has all the books from big publishers, the publishers that publish paper books too. What they do not have is the stuff from Kindle Direct Publishing. This is stuff people have self-published on Kindle, I think Amazon requires exclusivity when you do that too?

In my opinion this is actually more of a blessing than a curse since I consider 99% of that garbage I don't want to sift through.

Sometimes multiple store headaches are still a necessity depending on region. Even on Amazon.