Exactly. We never do. Basically, any real assessment of a club's draft performance is historical - evaluations that try to look at recent drafts are inherently subjective - based on scouting reports, observations, projections, etc. They're not based on career performance data, because most of that doesn't exist yet.
It's straightforward to look at a Phillies draft from 15 years ago, and compile the total career WAR of the players drafted - and to compare that kind of data across teams. The data are there, they're basically complete; it's trivial to analyze.
But the past three years? There's no data. There are opinions, there are subjective assessments of scouts, of sportswriters, of fans.
Obviously, an organization cannot wait 15 or 20 years to evaluate its employees' performance. Clubs must evaluate draft performance using information available, without waiting for career performance data. They have to use what they've got.
Fans (obviously!) try to do the same kind of evaluation - with even less information than the clubs have. For example, clubs will have particular development objectives for recent draftees - objectives which may well never be publicly articulated - and there's no way for fans to evaluate player development against those objectives... so we make assumptions about what the goals are, and evaluate against those assumptions.
Bottom line: We can really only form subjective - tentative - evaluations of recent draft performance. There's a lot we don't know. No reason not to make such judgments... as long as we keep in mind that they're preliminary, tentative, and don't start convincing ourselves that we know more than we really do. (How often have we seen fans assert "I still believe that..." when subsequent data contradicts a previously-held position? Saying "I was wrong" is hard for any of us! 
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