Why Rimuru is strong? Well it due to this:

-A will and soul strong enough to cross over worlds (cardinal world) when dying and reincarnating without losing its memories.

-Being named by a true dragon (a race that was a pinnacle of existence) and sharing its surname that recognize him as an equal

-A being that would get stronger by eating powerful beings (etc) and by its subordinates evolution

-Have a skill that can manage the laws of the world aka Wisdom King: Raphael (it evolved into a manas (Ciel) that far surpassed it)

-Busted with abilities

-etc

So Rimuru having so many abilities, have a soul that was strong enough to possessed it, and have a manas that can manage all of his abilities to the full extent, and was evolving, adapting, or getting stronger due to it.

-Lastly he stayed in the void a very long time and have all the time (there's really no time in the void) in the world to become more powerful

This explain why he is so strong, there was a lot of speculations about who or what really Rimuru is, but I think there's no need to think that deeply about it when you read what I said.