Ben – in a word, aesthetics :)
While your method is simpler and may lead to a better finished product, it is nearly impossible to gauge the amount of foam needed to fill the ball without it billowing out and all over the leg and workspace. I had used foam in the past that stuck tenaciously and badly stained anything it touched. Based on that experience I choose the method I documented. Truth be told, the particular Dow foam I used on this project didn’t stick well and also didn’t seem to stain. Another reason is that the hole in the ball will need to be enlarged enough to fit the fill tube…I chose to make the hole as small as possible.
I had planned to experiment with the process you described if I ever had to rebuild the feet.