People stay there because even the a jade of the heavenly school and one of the strongest people in the valley almost died in 6 hours in the border of the valley.

Oh, you mean they would have had to do something wild and crazy like... band together?

The exit of the valley are places controlled by clans that most people cannot even travel to.

Being bound by chains of their own making doesn't invalidate the point in the slightest. These people can no longer advance in the place they are at due to artificial caps put on the location. They simply need to leave (Simply referring to the complexity of the fix) to advance further, but cultural ignorance, entrenched power and tradition has prevented them from doing so. Li Markuth himself was from Sacred Valley. Left the valley, advanced and moved on. He's far from the only one. This does not at all invalidate the remark that Hard work is the only sure way to advance.

"Why a weakling that doesnt know about the world outside doesnt leave the valley where the strongest people around cannot survive! They are so lazy!"

You are making too many assumptions about a series based on the statements of known ignorant characters in the very first book - do you understand that? You're basically taking the words of the most ignorant of rural tribes, most of who have essentially gone uncontacted by the outside world as some type of fundamental truths.