"Why does Trisquel not use Linux Libre by default?"
In a way, it does. It uses a modified version of the Linux-libre deblobbing scripts (those which are used to remove blobs from the kernel from kernle.org) to also catch the modifications done in Ubuntu-land.
"Isn't it easier than deblobbing Ubuntu's kernel which is probably full of proprietary garbage?"
It might be, but the difference in work is not much - It's not like every kernel update need to be re-deblobbed from scratch. Rather, it's typically a one-time update of the deblobbing script for major kernel releases and it generally continues working just fine from then forward until the next major release. In addition to it not being a big difference, not using the kernel from Ubuntu would also mean that Trisquel doesn't get Ubuntu's other (free) kernel patches and wouldn't get Ubuntu's security updates. These LTS releases of Trisquel are supported for five years, and most kernels are no longer supported by kernel.org during that time so Ubuntu's kernel team manages it. So using Ubuntu's kernel is really the best choice.
