If I make ten games, and you make one, you could say I have a ten times greater chance of being published.
Sure, your one game might be better than the average of my games, but I still probably have a five times greater chance of being published. This doesn't mean I complete all these games. I abandon most of my games early on.
Game design is really hard. I don't think anyone really knows exactly how to make a great game.
My publishable games have not come from some stroke of genius, but from a lucky choice of theme or mechanic, plus a lot of highly skilled polishing.
Each new game is another stab at greatness.
If you only ever make one game, you'll never make a great game.
There's so much that needs to be learned, and it can only come from a variety of experiences, successes, problems, and failures. Spending all your time on one game doesn't give you that necessary experience. Even if you did become an excellent designer later on in the development of your game, it would be too late to apply this knowledge to the important early choices you made for your game.
If you made a second game, wouldn't it be better than your first game?
When I look back at my early failed prototypes, I can see mistakes in them, that I couldn't see at the time. Until I made my 37th game, I didn't understand goals properly. That alone probably killed most of my early games. There are likely all kinds of things I still don't know.
I made a pirate game. I put a lot of time into it. I couldn't get it published. Was it a bad game? No. Was it about pirates? Yes. I didn't realise how hard it was going to be to get a game with no new theme or mechanics published. So, it went back to my shelf.
A friend spent years making a game clearly evoking the Power Rangers, in everything but name. While we were working on the game, the company that owned the IP suddenly announced a series of Power Rangers board games.
I've seen many people's "masterpieces", and just thought to myself "clearly this isn't going to work", or "There's really no audience for this." That person might be you.
When I play another designer's game, I rarely quibble with the details. The game has huge issues, that the designer is completely blind to. Even if it's good, the game needs major structural work. Other games are feeble, and unsalvageable.
Some people seem to think that the way a great game is made is by working on one idea endlessly, until it's great, and that anyone can make a successful game. The correct way is to just keep trying different ideas, until you discover a good one.
Even if you don't make any mistakes, it doesn't mean your game is great. The central idea of the game might just not be that good.
There are so many mistakes and misfortunes that can befall you, that a single game has a good chance of being doomed, due to outside circumstances, or due to flaws you cannot see or understand.
Make lots of games.