It quite often is actually. Until today, about half of all human genes have never been described in any detail at all. We do not know what function they serve, only that they exist and that most of those are probably not that important. Therefore no early career scientist can really dare to investigate any of them, if they do not certainly want to endanger their future careers. If they would have not worry about that, a lot of bright minds that now only study those 5% of genes that promise the most interesting results could diverge and with a high probability discover a new PD-1 here and CTLA4 there. And thats not a question of time (besides the maybe 10 years to study a gene well), that is 100% a problem of funding.