Bipolar I is often referred to as more severe just because the criteria for Bipolar I include full manic episodes, while by definition Bipolar II has no full mania but only hypomania. The first time someone with Bipolar II experienced a full manic episode, their diagnosis would change to Bipolar I. The criteria have nothing to do with how serious the depression can get or how profound the impact of the disorder is on each person's life. Both include depressive episodes as well. Mania is considered more severe than hypomania because people's judgment and behavior are more impaired with mania than hypomania.

I can understand how those statements can feel invalidating. I've been diagnosed with dysthymic disorder several times, and I felt like that category completely overlooked the fact that I had continuous depression that lasted most of my life with no remission and that I also do experience major depressive episodes. A dysthymic episode would be considered less severe than a depressive episode because not all of the criteria are met (at that time) for a depressive episode, just like a hypomanic episode doesn't meet all of the criteria for a manic episode.

What none of those diagnoses are even meant to do is to define the degree to which any individual is affected by their symptoms. Each person is different, and I do agree with you that often people with the "less severe" diagnosis have more persistent symptoms - just being that way more of the time, and it does seem like that ought to count for something too. Maybe it would be better, rather than the labels we have, to have a visual representation that shows like a thermometer where each person's mood sits and their individual range of moods, as compared to whatever normal is.