Why didn't they stop with Rocky 4?

I am a big fan of the Rocky movies; I've seen them all. We recently watched 'Rocky Balboa', the last and perhaps final installment in the series. In my humble opinion the following aspects make a movie great:

1. It helps me change the way I think
2. It makes me laugh out loud
3. It motivates me to do something

'Rocky Balboa' did none of this for me, but since I'd seen all the others (including crappy Rocky 5) I had to see this one. The storyline was predictable, the dialog weak, and I hope I never see Sly Stallone with his shirt off again. Perhaps it only seems less than stellar because I'm secretly comparing it to one of the greatest films of all time, Rocky 4.