Well you asked "why" and you were answered. Whether it's a good system or not, well that's subjective.

Personally I find that even without asking for an invite there are usually PF up you can join without asking, or you can make one and incoming people will join it, so it's not really a big deal (and because of that I doubt the devs will spend the effort needed to fix something that can be so easily circumvented, regardless of whether it's good or not, especially when there isn't a general concensus that it's bad.)

Doing full tank enmity (or doing 1 hit as a healer and then going full heal spam) is one way to get full or near-full credit solo, though it's not guaranteed.