You are mistaken in thinking they use the Linux kernel in ESXi. There is no Linux kernel anywhere in ESXi.
They have written their own operating system from scratch, and they did a complete rewrite of the kernel in the update from ESXi 3.5 to 4.0.
What they have done is copied a subset the interface API from the Linux kernel.
Much how like the Wine Project has copied API details from Win32 without permission from Microsoft.
This allows existing driver source code that already works in Linux to be compiled using the VMware driver development kit
into a binary that can be loaded as a driver in ESXi.
This means that hardware vendors can write the driver once, and then it could be built for either Linux or ESXi, so that seems beneficial for Linux users to have more drivers still being written for Linux.
This is considered a legacy framework, and VMware is already phasing this out... see details on the new native driver framework
This will be sad, as the native driver framework is proprietary, and it will likely no longer be possible to write your own drivers for ESXi, once vmklinux is gone, without purchasing the driver development tools at high $$$.
Also, major enterprises are running ESXi on much of their hardware, so the incentive may go away for many manufacturers to
release information or develop Linux drivers;
they can just produce their binary ESXi drivers and be done with it.