Uh because he wanted the car on a level surface.
If you have something with an even surface, thats ideal so the lubricant doesn't flow everywhere.
On ramps the way the lubricant leaks is going to be unpredictable and it will eventually shift and some of it may not come out.
Jackstands on a level surface, both raise to the same level, is ideal.
Even more ideal is a lift.
...and best yet if you're a small guy like myself, then just crawl under there and the car will naturally be level.