It's my main opening... and I see no reason to change that, I think it is phenomenon and unmatched on multiple levels honestly. Alot of GMs advocate for it - Nakamura, Simon Williams, Hans Niemann, others.
I thinks it's unpopular due to the past state of theory, which formed the present day conventions... the fact is most people copy one another. In the past 1... d5 was considered the "problem" with it. But after 2. d4 you can reach a Jobava, which is completely viable at high level and not a problem. In the recent past people also insisted on vying for theoretical or engine advantages with white more than they do today... as chess has evolved lines have become played out, people are looking more to sidelines to get an edge / get the opponent out of book. People have also evolved in how they use engines, seeing the engines more as a tool while still focusing on the human factors, not necessarily playing the Ruy Lopez every game just because it gives the maximum engine advantage for example, but rather developing offbeat repertoires using engines and focusing on playing the opponent rather than a computer.
The Jobava only really became popular and acknowledged as a good opening for white around 2016 when Jobava beat Topalov and a few other high level GMs with it / Rapport started using it. That said, nowdays I do see more GMs talking about 1. Nc3. Simon Williams has a chessable course on the Jobava where he promotes 1. Nc3 as an alternative move order to 1. d4.
The only argument against reaching a Jobava from 1. Nc3 is that the opening is quite large compared to the typical 1. d4 chigorin setup. However, that's more of a remark on the chigorin itself than on the Van Geet - the chigorin might be one of the most compact repertoires out there. After 1. d4 / 2. Nc3 the only really viable alternatives for black are the french, caro-kann, and pirc. Infact, you can transpose the french and caro-kann into a Jobava-like positions, often back into the Jobava. There are a few other chigorin sidelines like the Alburt defense or Irish gambit but these are rare. But it's a very compact repertoire and usually you will just get a Jobava from it.
The Van Geet on the other hand has the same sort of variety you see in other openings like 1. e4, 1. d4 > 2. c4, 1. c4, 1. Nf3, etc.. This can be beneficial in a tournament setting as your repertoire will be less predictable. 1. Nc3 is also a very flexible move - in most cases you have a choice of transposing into an e4 line, a d4 line, or a unique 1. Nc3 line which are some of its best lines. But for an online repertoire... I see it a different way - it's not that I "have to" play some of these Van Geet lines such as the Napoleon attack or the Two Knights Sicilian - it's that I get to play some offbeat and completely brutal lines. If I don't know the lines perfectly... I'll know them better than the opponent. This combined with the confusing transpositional nature really just increases the lethality of your repertoire.
The other nice thing about the Van Geet is.. the common e4/d4 lines it can transpose into are also generally good lines - i.e. the Vienna game, the Closed Sicilian, the Paulsen French, the philidor (I played someone earlier today who tried to avoid the Napoleon attack and accidentally found himself in a philidor).
So overall I think it is just something that's been overlooked and theory has not caught up with.
After seeing all the rich tactical possibilities for the Dunst opening, (1.Nc3) also called the Queen's Knight Attack, I am surprised that the Dunst is labelled an unnusual opening, along with the Grob's Attack (1.g4) and other unusual openings. Are there any current grandmasters who still regularly play the Dunst? Also, there seems to be a dearth of information on this opening besides a good Wikipedia article I found which reads, "
The reasons for 1.Nc3's lack of popularity are that it does not stop Black from occupying the center (unlike 1.Nf3, which prevents 1...e5), and it blocks White's c-pawn from moving, thus making it impossible to play c3 or c4 (which are often desirable moves) without moving the knight first." (Wikipedia)
However, the Dunst also transposes into many other openings, and offers great flexibility, and sharp tactics if white knows what he is doing.