Because I don't perceive any one form of autism as inherently more disabling.
Individuals may have lower or higher support needs of course, (and this can fluctuate in a person over time, too) but at a category level it's genuinely not possible to say "this is mild autism" "this is severe autism" "this is profound autism".
So I disagree with your "lost a toe" Vs "lost a leg" analogy I'm afraid. How do we measure that? In the impact on the family? On the impact on the carer? On the person's quality of life? And if so, according to whom?
My son is what you would probably call "lost a toe". He is academically very able. Was in mainstream school all through. But the impact on him of operating within the NT world was cumulatively enormous. He had, as I have said elsewhere, a complete breakdown. 5 years later whilst he is no longer in imminent danger of suicide, he spends his days in his darkened bedroom and is entirely reliant on others to advocate for him. I run his finances, his universal credit applications, his pip applications, organised his limited capacity for work assessment, feed him, have to chase him to wash himself, have given up on him ever cleaning his teeth, have to wash his clothes, and expect him to be dependent on us forever. He's in his mid 20s. I don't feel like that's mild. And I work with tens of families with similar experiences every year.
On the other hand, my father found his work niche in an obscure field and in his 80s still works 7 days a week. He found his coping space in work. Is that mild? I don't know. It's different from what my son went through, and it's different from parents whose youngster is smearing feaces from a nappy in their teen years.
I suppose that is why some kind of explanation of the profile within autism rather than "support needs" or "severity" judgement is what I think would be most helpful.