So the question you need to ask is 'Who decided "latest stable kernel" is 3.18?'. Wat that decided by The Kernel People or by Canonical? (the answer: the Kernel People).
Why Ubuntu ships its own kernel instead of latest stable build?
The Ubuntu kernel configuration files need to be added, tested and tweaked to the new kernel. That will cost time and resources (the idea is that it is better spent on the next Ubuntu release). So the latest stable release according to the Ubuntu Kernel Team is currently 3.16.
Kernel/dev/extended stable and mainlinebuilds should be an interesting read.
We will get a kernel update every 6 months that is tested and stable so it is not a long wait (less if you assume the kernel release is not out on the day a new Ubuntu appears).
How many end-users do you believe are waiting for kernel 3.18? Unless you are a developer (and you will know how to upgrade the kernel; and you will know where to find an upgrade method) there should be hardly any need to upgrade your kernel. Yes, there are some interesting features in any new release but those are not to interesting for the mainstream users of Ubuntu.
And we will get those features in 3 months anyways.