Why is 1. Nf3 not nearly as popular as 1.e4 and 1.d4? The opening move 1. Nf3 is considered as a perfectly sound opening and it is popular, but not nearly as popular as 1.e4 and 1.d4. Here are my thoughts about this:


In most openings, white will make the move Nf3 sooner or later. Since white needs this move (in most cases) anyway, why not make it the first move?
White has much flexibility with 1. Nf3. Here are a few examples:


For 1. Nf3 Nf6, white may reply with either 2.c4, transferring to the English or 2.d4, transferring to Queen's Pawn Game.

For 1. Nf3 d5, white may continue with 2.d4, transferring to Queen's Pawn Game.

For 1. Nf3 c5, white may reply with either 2.c4, transferring to the English or 2.e4, transferring to Sicilian. 

At the same time, white is not afraid of some weird replies such as 1. Nf3 e5 as replying this with 2. e4 is perfectly fine for white. Capturing the pawn may be fine for white as well.



Why then is 1.Nf3 not nearly as popular as 1.e4 and 1.d4?
It is funny since 1.Nf3 scores better than any other opening move per Mega 2020 at 55.5% winning percentage for white, compared to only 52.8% for 1.e4, 54.3% for 1.d4, and 54.4% for 1.c4.

I think a lot of it is psychological regarding the types of positions that can arise. 1.Nf3 is the most fluid of all the moves I mentioned, and I think that players simply like more concrete play than not. It also reduces what you need to memorize, or understand, if you go straight into a specific pawn formation with d4, e4, or c4.

I also think that at the lower levels, that fluidity can lead to positions that players, both players, but since it is white's choice, that he/she specifically is not as comfortable with since he/she can end up on his/her own in a hurry. You need to know A LOT more about transpositions into other openings, which is a major undertaking. The play is also more complicated, which drives weaker players away more.

At the upper levels, meaning 2700s, I do not believe that they are afraid of getting lost in the opening as easily, but there still may be an underlying desire to play 
"their" openings, so they play more concretely.

One thing I do not believe is that after 1.Nf3 that 1...e5 is a valid response. White, having played 1.Nf3 probably likes d4/c4 openings, so is not likely to go into a double-king-pawn opening, but still has a great response:

 [FEN ""]

 1. Nf3 e5? 2. Nxe5!


Thanks for the gumball, Mickey! (from an old U.S. commercial for a Mickey Mouse gumball machine)

Based on my discussion with Phonon below, I am adding this chart that shows the flexibility of each main opening move that aims for the closed openings (d4/c4/Nf3). At the higher levels, this is a main reason, but at lower levels, I still believe that it is more about psychology, and believing that immediate occupation of the center is crucial. At lower levels this also can often put your opponent on unfamiliar grounds,