it is because you have 15 ships.  Military rating has to do with overall attack and hitpoint values, not the "untouchable" status of your ships.  Most likely the AIs have a lot more than 15 "paper tigers".  You may actually be in the lead, despite the graphs.

But, if you aren't having fun, play something else I suppose.

Other things to possibly look at:  How is your population (main source of raw production)?  Are you creating good focused adjacency hubs on your planets?  You mentioned all your citizens are leaders, but what about administrators (for more colonies / starbases )?

I love leaders, but they only become really useful when you have a lot of planets.  You get more gain from putting a scientist (or whatever) directly on a planet, for a 30% bonus instead of 6% per planet, until you have enough planets to justify.  Also, on-planet citizens level up over time, so continue increasing the percentage bonus.