Because Xournal doesn't have "line" unit.
It's a drawing...

How do you know how much big you write or how high a "line" of handwritten text is?

It's fine to insist, don't worry.. it's just that it way too complicated to come up with a "line height" personalized to your own style of writing.

I keep thinking at solutions and welcome you to do the same...maybe one of us will have a striking idea that works for everyone