Game balance. To go beyond 11 levels, developers would have to create, test and balance new perks and design enemies and other challenges to match those levelled player controlled characters. That would cost time and money (of the developers). Besides, it’s point of no end effort. Let’s say they would do it and extend content to say lv. 20. Next day you would come complaining that you can’t progress beyond that.
There is always going to be some limit. Lv 11 is as good as any.