Why is e4 f5 mistake for Black???

@3
With white it is less severe because of the extra move, i.e. tempo.
Nevertheless: 'It loses by force' - Fischer, 'I could not find a way to equalise as white' - Kramnik
'6.3% white wins, 23.4% black wins' - AlphaZero Figure 4. (d)
https://arxiv.org/pdf/2009.04374.pdf

@tygxc Yes, we do lose a pawn, but they lose their central pawn and we will easily get two central pawns and get dominant control of the centre if played nicely. We do weaken our king and pawn structure, but we can always castle long.
@5
"we can always castle long"
++ How do you get Bc8 out of the way? The white pawn on f5 hinders the development of Bc8.
"we will easily get two central pawns "
++ How do you get a central pawn to e5? The white pawn on f5 prevents that as it takes en passant.

@tygxc Yes, we do lose a pawn, but they lose their central pawn and we will easily get two central pawns and get dominant control of the centre if played nicely. We do weaken our king and pawn structure, but we can always castle long.
You lose a pawn, and that is that.
You get nothing in return after 1.e4 f5? 2.exf5 (already threatening Qh5+ which will force you to develop your king instead of your pieces) 2...Nf6 3.d4 d5 4.Bd3 and you have not dominated the center, you are a pawn down, and you have a few weaknesses on the kingside.
But if your intention is to create a trolling thread, then sure, 1...f5 is a brilliant move.
If you really like to play ...f5, then delay it to move 3:
https://www.chessgames.com/perl/chessgame?gid=1640388

It is called the Duras Gambit. It is rather dubious, but fun to play in bullet to get opponents out of their theory.

@6 we first somehow gotta take the white pawn on f5 to get two central pawns. When we play e4, its a discovered attack on the pawn by our bishop and the white pawn can't be defended unless white defends it by ruining their pawn structure. So Bishop on c8 is not stuck. Guys, please tell me if I should be playing that anymore in my games.

@6 we first somehow gotta take the white pawn on f5 to get two central pawns. When we play e4, its a discovered attack on the pawn by our bishop and the white pawn can't be defended unless white defends it by ruining their pawn structure. So Bishop on c8 is not stuck. Guys, please tell me if I should be playing that anymore in my games.
Yes, the answer is yes. You should play that anytime you are Black against me OTB.

At the rating you're playing at, it might work. Even so it will stop working when you start to meet players who are more adept at exploiting your mistakes. If you check the ratings of the people responding to your thread then it'll give you confidence that your opening wouldn't work against them. To progress it's sensible to learn how to defeat openings like this and develop an understanding of an opening which is more robust so can carry you further. Aside from the material, the first thing I'd be looking to do against a opening like this is preventing castling. If you can prevent your opponent from castling you have a good chance of being able to develop an increasingly stronger position while chasing their king around the board.

At the rating you're playing at, it might work. Even so it will stop working when you start to meet players who are more adept at exploiting your mistakes. If you check the ratings of the people responding to your thread then it'll give you confidence that your opening wouldn't work against them. To progress it's sensible to learn how to defeat openings like this and develop an understanding of an opening which is more robust so can carry you further.
In all seriousness, I think almost everyone who learns chess starts with the thought "I am going to play this really unique opening that no one has ever seen and will beat everyone!". Many times they get a bunch of quick wins against other beginners, but as soon as those opponents master basic opening principles and start to learn basic tactics, those "unique" openings fall apart.

@6 we first somehow gotta take the white pawn on f5 to get two central pawns. When we play e4, its a discovered attack on the pawn by our bishop and the white pawn can't be defended unless white defends it by ruining their pawn structure. So Bishop on c8 is not stuck. Guys, please tell me if I should be playing that anymore in my games.
By "we" you mean that you have company while playing?
And how the heck you are supposed to put a pawn at e4? You have to cheat and play ...e7-e4 in one go, else there is no way to achieve this.

Ok, I can't play that gambit. But what on earth is a good opening for a beginner newbie like me except for e5???

Ok, I can't play that gambit. But what on earth is a good opening for a beginner newbie like me except for e5???
I found that the French works extremely well at lower levels if you can catch the simple blunders your opponent will make

Ok, I can't play that gambit. But what on earth is a good opening for a beginner newbie like me except for e5???
There is nothing wrong with playing e5 after White plays 1. e4 (or even 1. c4, but that is a different situation). What you have to figure out is if you want to play the Ruy Lopez/Italian Complex with Nc6, the Petroff with Nf6, or the Philidor with d6. Each has their own sets of middlegame ideas. I would suggest pulling up some super GM games in those lines and see whose games you enjoy more and play those. For example, if you like how Nepo played in the Candidates 2 months ago, play the Petroff. If you like Carlsen's games from the 2016 World Championship match, Caruana's games as Black play the Ruy Lopez/Italian. It is harder to find super GM games in the Philidor, but if you like Simon Williams's idea in the Black Lion, play that.
If you want to play something other than 1...e5, you might try John Bartholomew's ideas in the Scandinavian as it will give you a fairly easy setup to get you going.