The most obvious: d5 is open diagonal for your g7 bishop, also there is no points for d5 so early, and it's matter of taste when to throw it, someone will play it, someone not.
Also after 3.d5 you can play against it without d6 - just play c6, Qb6 targeting b2.
By the way, if you play 2.Nf6 - it's not modern defence anymore. In modern defence black usually place his g8 knight on e7 square.
Check out variations on modern defence here
In the modern defense, after 1.d4, g6 2. Nf3, Nf6, why shouldn't white just push d5?
The most obvious: d5 is open diagonal for your g7 bishop, also there is no points for d5 so early, and it's matter of taste when to throw it, someone will play it, someone not.
Also after 3.d5 you can play against it without d6 - just play c6, Qb6 targeting b2.
By the way, if you play 2.Nf6 - it's not modern defence anymore. In modern defence black usually place his g8 knight on e7 square.
Check out variations on modern defence here
I will check it out, thank you.
white usually waits for something like ..e5 or ..c5 or nc6 or be6 before he plays d5. in the case of e5 or c5 white argues that black will no longer have both e6 and c6 as a means of chipping away at d5 which he probably must do as otherwise white just gains space for nothing. in the case of nc6 or be6 white gains a tempo, and usually d5 is only played if neither of these pieces have a good square to go to.
if you play d5 without a tempo and without first waiting for either c5 or e5 then you give black more options and you also lose time while now the space you gained is far less of an asset if it can be chipped away from all sides and in some cases black can even play around it. the time you lose can become more evident once black starts breaking down d5 with c6 or e6 eventually opening lines when the better developed side usually has the advantage.
on an unrelated note, the modern is not exactly easy to play and forgiving of even small inaccuracies. it also mostly conflicts with opening principles you are supposed to adhere to when starting out. i would suggest a more direct approach with 1 d4 d5 fighting for the center with pawns. even the more complicated 1 d4 nf6 2 c4 e6 and bb4(+) if allowed (and ..d5 if not) would still be simpler than the modern conceptually speaking.
white usually waits for something like ..e5 or ..c5 or nc6 or be6 before he plays d5. in the case of e5 or c5 white argues that black will no longer have both e6 and c6 as a means of chipping away at d5 which he probably must do as otherwise white just gains space for nothing. in the case of nc6 or be6 white gains a tempo, and usually d5 is only played if neither of these pieces have a good square to go to.
if you play d5 without a tempo and without first waiting for either c5 or e5 then you give black more options and you also lose time while now the space you gained is far less of an asset if it can be chipped away from all sides and in some cases black can even play around it. the time you lose can become more evident once black starts breaking down d5 with c6 or e6 eventually opening lines when the better developed side usually has the advantage.
on an unrelated note, the modern is not exactly easy to play and forgiving of even small inaccuracies. it also mostly conflicts with opening principles you are supposed to adhere to when starting out. i would suggest a more direct approach with 1 d4 d5 fighting for the center with pawns. even the more complicated 1 d4 nf6 2 c4 e6 and bb4(+) if allowed (and ..d5 if not) would still be simpler than the modern conceptually speaking.
Sorry, I meant 2...Nc6, why shouldn't white just push 3.d5 vs 2...Nc6?
After 1 d4 g6 2 Nf3 Nc6 white should play 3 d5 with a winning advantage.
What higher level game are you talking about?
"After 1 d4 g6 2 Nf3 Nc6 white should play 3 d5 with a winning advantage"
"winning" may be a slight exaggeration, but White sure gets a fine position with no effort. But nobody plays the Modern that way, do they? 2...Nf6 is the overwhelmingly popular choice, followed by 2...Bg7 and 2...d6. 2...Nc6? is chosen about one tenth of one percent of the time

I play the modern defence pterodactyl and beef eater lines because I’m a Sicilian player who preferably wants to avoid the KID.
Sometimes I am fortunate to transpose to the hyper accelerated dragon line by move 3. c5, in that case, I’m enjoying the game.
I would at least look at the pterodactyl and beef eater lines if the modern defence will be your main reportoire against d4. This way you see the imbalance, and with imbalance, you both naturally get opportunities to play for.
Somebody mentioned playing 1.d4, d5, that is the best advice. Playing the modern defense breaks all the rules for a beginner.
Hello, I am a budding student of the modern defence. I am always reluctant to play Nf6 early against 1.d4 because I feel like my knight will just be harassed away with 3.d5 or even blocked with 2.d5, but looking at higher level games this almost never happens. White just lets the knight be and continues development.
Why is that?
Maybe this is too broad a question but I have struggled with this for some time and I can't figure out why, maybe someone here can help me clarify it so I can be brave enough to get my knight out early in to the game.
Thank you.