I don't know. The range of the Scale effect predates me joining this team. Regardless, if you haven't already done so, please feel free to submit a feature request: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform
"Why" is something I am not certain that anyone can answer. Just keep in mind that Premiere Pro doesn't use vector graphics unless you are trying to do this with text in a title. There is a different workaround for that. So anything you zoom in on will be fairly well pixelated.
How to get around it? Well, you can get 300% using Transform on top of Motion, so that should come out to about 2400% total.
Does that help? Just zoom in to 600% in Motion and then search on Transform. Use that effect to zoom in farther.
If you are trying to zoom through text, you might simply want to create your title as large as you can. For a 16:9 title, I believe that is around 10240X5760. If you start with text that large, and scale it down to fit your 1920X1080 frame, for example, you will not need to zoom quite as much past 100%. The quality will be superior to just zooming through a 1920X1080 title.
If you want to zoom in to a very high number.
Zoom all the way in, then nest.
Repeat zoom on nest etc.
Thank You Steven & Ann. I'll try it.