Lots of beginners are out of their opening book after e4 e5 (or d4 d5), and just play what they think looks ok. I get the Marshall Defense all the time against my Queen's Gambit for example (1. d4 d4 2. c4 Nf6 3. cxd4 Nxd4 4. e4) which is an inferior response compared to the QGD, QGA, Slav, or Chigorin even. Not to mention <1400s playing something other than d4 after 1. e4 e6, or opening with something like 1. e3... Lower levels often just don't know any opening theory.
Why is Philidor's Defence so popular at < 1300 levels?

you dont lose a pawn after 3.Bb5 in the ruy lopez. If 4.Bxc5, dxc6 5.Nxe5 then 5... Qd4 forks the Knight and pawn. After the retreat 6. Nf3, Qxe4 winns back the pawn with advantage to black.

you dont lose a pawn after 3.Bb5 in the ruy lopez. If 4.Bxc5, dxc6 5.Nxe5 then 5... Qd4 forks the Knight and pawn. After the retreat 6. Nf3, Qxe4 winns back the pawn with advantage to black.
But beginners don't know that. Their memorized openings don't extend that far. I agree with the people above that Philidor's Defense is played because it is an obvious response and beginners don't know book that far.

I've been surprised seeing it, as it is seen as less sound than 2...Nc6. Perhaps this book made it more popular: "A Cunning Chess Opening for Black: Lure Your Opponents into the Philidor Swamp!" by Sergey Kasparov
So I was going over my games and noticed that 30.6% of the time when I (as white) play 1.e4, e5 2.Nf3 my opponents have played 2...d6. While 2...Nc6 is still the favourite reply at 47.5% the philidor is the closest competitor. ... So this got me wondering. Why is the philidor such a popular response at <1300 levels? ... Was there a very popular book/video/article released? ...
These books have been published in the last few years:
The Black Lion, 2nd Edition by Jerry van Rekom & Leo Jansen (2009)
https://web.archive.org/web/20140627122350/http://www.chesscafe.com/text/hansen123.pdf
A Cunning Chess Opening for Black by Sergey Kasparov (2015)
http://www.jeremysilman.com/shop/pc/Cunning-Chess-Opening-for-Black-A-76p3899.htm
https://www.newinchess.com/media/wysiwyg/product_pdf/9008.pdf
However, I would think that a reader of such a book would be playing 1...d6 instead of 1...e5. My guess would be that a <1300 2...d6 player is just trying to avoid 2...Nc6 for the sake of variety and perhaps to avoid an opponent's experience with 2...Nc6.

After 1 e5 e5 2 Nf3, Black can not be sure of getting the Four Knights Game. 2...d6 is a way to avoid much of 2...Nc6 stuff. I suppose 2...Nf6 is another way to do that, but I would guess that a lot of beginners do not know about that idea and have never thought of it.
Harrwitz did not seem to be too concerned about that.
http://www.chessgames.com/perl/chessgame?gid=1055947

I'm a very low rated player myself. barely 1000, but I don't see the Philidor quite as much as you do. Only about 20% of my games that start 1. e4 e5 2. Nf3 are replied with d6, with the next lowest being Nf6 at 13%. I think it's more popular at lower levels because of how passive it is though. While not exclusive to the Philidor, passive openings for black seem pretty good at punishing over aggression in white. And I think low rated players are more prone to attacking without thinking.
But I've found that so many players, at least at my rating, play the Philidor super weird, and after the exchange on d4 they immediately start trying to attack with either c5 or something equally aggressive and I don't get it. I always end up with an odd position. This is one of my most recent games in the Philidor as white so you can see what I mean.
Lots of beginners are out of their opening book after e4 e5 (or d4 d5), and just play what they think looks ok. I get the Marshall Defense all the time against my Queen's Gambit for example (1. d4 d4 2. c4 Nf6 3. cxd4 Nxd4 4. e4) which is an inferior response compared to the QGD, QGA, Slav, or Chigorin even. Not to mention <1400s playing something other than d4 after 1. e4 e6, or opening with something like 1. e3... Lower levels often just don't know any opening theory.
I bet I can beat you with the Marshall Defense. 1.d4 d5 2. c4 Nf6 3. cxd5 c6. is perfectly playable. 3. Nxd4 is a mistake imo.

Oops, yeah I had my notation wrong. I'm sure it's playable (especially without 3 ... Nxd5), but I've never seen the 3 ... c6 variation. At any rate, there are several more solid options, which was my point.
So I was going over my games and noticed that 30.6% of the time when I (as white) play 1.e4, e5 2.Nf3 my opponents have played 2...d6. While 2...Nc6 is still the favourite reply at 47.5% the philidor is the closest competitor. The two next replies don't even come close; 2...Nf6 (6%), 2...Qf6 (5.5%).
So this got me wondering. Why is the philidor such a popular response at <1300 levels? I dont follow professional chess so has there been a top player playing it? Was there a very popular book/video/article released?
Thank you in advance!