Sorry, I missed that question.
The simple answer is that they don't tend to do the same thing, and they don't do the same things well. Amway does a much better job of, for example, teaching knowledge about the products. This isn't surprising given they developed them.
The external companies tend to do a better job in the motivation/leadership/personal development etc areas. Amway used to do this stuff as well, but the "competition" did it better and Amway effectively decided to step back in this area. One problem, particularly in the US over the past decade or so, was that they also pulled back in the product training area, which the 3rd party companies do not do well, and the business suffered as a result. You can't sell the products on price,you have to market the features and benefits. Too many people never learned this, so instead they have to market the products on the basis you can make money as a business - this can (and did) predictably lead to exactly the kind of "pyramid" claims coming here. Some folk were buying products not because they wanted them, but because they thought they could make money doing so. It's a silly concept and it doesn't, and didn't, work. A number of field leaders got kicked out of Amway a few years ago for taking this too far, stretching (Even breaking) Amway's rules to the extent it *did* verge on illegal pyramid territory.
Amway has realised their mistakes and has developed far more product training again, necessary IMO to build an Amway business profitably and sustainably.
The 3rd party companies still focus on motivation/leadership/personality development.
It's important to note that by far the majority of people who join Amway never use the 3rd party companies. It's also important to note that by far the majority of Amway's volume can ultimately be traced to people who are.