It's because you guys are so supportive that I'll remind you, you guys are getting frustrated over nothing
As I can assume most of you must have forgotten what kind of system Michael has
Knowledge System
The more knowledge he has the faster his growth
The less knowledge he has the slower his growth
Get it?
It's ridiculous to the point that if he was to find a tier 20 library, with books from Tier 1 to Tier 20 he would reach Tier 20 that same day
But as you can see the world's he's been to are heavily lacking in higher Tier knowledge, hence why his women ( who he trains with all the knowledge he gained Tier 1-5 can reach Tier 5 ) Their growths also halted at Tier 5 as they now have to feel in the dark like he had to in the beginning, the faster he grows the faster they grow
* Spoiler *
Highschool DXD is a Tier 15 world
* wink wink *