[2021] What is the point of unlocking base skills by putting points into the base passive tree?

Not a lot would change if these skills unlocked with character level instead of points in the base class tree. The reason why they’re unlocked the way they are is to introduce the mechanic earlier and because it provides an extra reward or motivation for reaching that tier of passives.

I am assuming the rest, instead of just having fewer skills than other classes, are simply not in game yet.

This is correct, the current plan for release is to have 12 skills for each base class and 5 for each mastery.

I think most people would prefer 4 more skills instead of some text and a line

Adding a skill takes a lot of work. The main thing preventing us from adding 20 new skills (4 for each base class), is definitely not the existence of that text and line. In future, when we want base classes to have more than 12 skills, we will simply adjust the skills panel UI to fit them.