Why does Samsung use an older version of the Linux Kernel (for its Android version)

Samsung is shipping kernel version 3.4.0 with Android 5.1.1.
https://forums.androidcentral.com/e?...token=6zbiaIVW

But, according to this, version 3.4.0 is dated back in the Jelly Bean era:
Which Android runs which Linux kernel? - Android Enthusiasts Stack Exchange

My question is, WHY?

Only Samsung can answer that. Stability? "It works, so why spend money rewriting a newer one?" Supporting a version costs money, supporting 2 versions costs twice as much? There could be many reasons.

They did change. After June 2, 2014, they came out with a later mod of 3.40, because someone had come up with a root exploit that used a bug in the kernel.

Looks like their Marshmallow builds are running 3.4.0 too:
Deja Vu: Another unlocked Samsung Galaxy S5 unit accidentally receives update to Android 6.0.1

they avoid to rewrite touchwiz and other app from samsung.