Except...if you did that, then - by definition - both conditions would be incurable, permanent developmental conditions.
The problem is that many autistic people's experience is that some days they can be level 1 (to take the insurance-based DSM definition, which is functionally identical to the old Asperger's definition), and then other days they can be level 2. The proportion of these days will often change with age; their level 2 days may be 1-5% of the time when young, increasing as far as 50% of the time in their 70s.
Similarly, there are people who move between levels 2 and 3. It's rarer for variation between levels 1 and 3, but not unheard of; there are plenty of reports of children diagnosed at level 3, non-speaking and unable to interact with the world, who suddenly start speaking in full sentences between 7 and 10 years old, and by the time they're in their late teens their support needs have reduced considerably to the point where they can deal with mainstream education and even hold a job when they're older.
This begs an important question: how, medically-speaking, would it be possible to have one incurable, permanent condition on one day, but an entirely different one the next?
This is a big part of why they are all considered part of the same condition, the other being that they have the same underlying cause. The position that they should be considered distinct conditions is a social one (usually demanded by people who are not autistic, and therefore have limited knowledge of the condition), not a science-based one.