I hadn't realized the common consensus was "yes". I am inclined to agree with you. They work together thematically, and carry the exact same disaster, but the dates are so far apart that I have actually always just assumed they didn't have connected continuity.

In Dawn of the Dead, though it was made ten years after the first film, and the setting of both films reflect the time they were made in, it seems the zombie apocalypse had happened very recently. Though admittedly, Day of the Dead obviously takes place a good deal of time after the outbreak began; I guess it would be reasonable to assume that Dawn and Day share the same zombie apocalypse. Bottom line is that I don't think it really matters. The details of the outbreak were never even a major point of those films, they all drop right into the thick of it and focus on the characters in the situation. The audience is purposefully kept in the dark as much as the characters are, and the details are left vague, because they aren't that important to the story being told or the points being made.

Not to say discussions like this arn't still fun to have.