user34315: In the Ruy Lopez, Morphy Defense Main Line,
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7

after 4.Ba4, the engine suggests Be7 as one of the best moves, tied with Nf6. The opening book Perfect2021 confirms this. Yet, there's no mention of this move order in any of the materials I've read so far, including the Wikipedia article and the Chess Opening Theory Wikibook chapter on the Morphy Defense. Lichess' Masters Database has only 132 games where 4...Be7 was played, out of 88,219 (0.15%). What am I missing? Why does no one play Be7 first?

user18809: 5...Be7 will almost always transpose into 5...Nf6 followed by 6...Be7, so there's no special line to worry about. However after 5...Nf6 Black still has other options
We usually want to develop first the pieces that we know which squares will go to. The knight goes to f6 on almost every game in this line, but the bihsop can decide later between c5 and e7.

user4418: 
Why does no one play Be7 first?

Because Nf6 is more forcing. It attacks the pawn on e4 forcing white to do something about the threat. Castles indirectly does this by opening the possibility of Re1 if black takes the e4 pawn.

user8860: Practically/psychologically Nf6 makes White's life a bit harder because it also leaves open the possibility of following up with Bc5 (I see that this is called the Møller defense).  This kind of thing is part of why it's a general opening principle to develop Knights before Bishops: usually we know where the Knight goes (here, to f6) while the Bishop has a choice so we keep more options by moving the Knight first.

34315: <p>In the Ruy Lopez, Morphy Defense Main Line,</p>
<pre><code>[FEN &quot;&quot;]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7
</code></pre>
<p>after 4.Ba4, the engine suggests Be7 as one of the best moves, tied with Nf6. The opening book <a href="https://chessengines.blogspot.com/2021/01/new-chess-opening-book-perfect-2021-abk.html" rel="noreferrer">Perfect2021</a> confirms this. Yet, there's no mention of this move order in any of the materials I've read so far, including the Wikipedia <a href="https://en.wikipedia.org/wiki/Ruy_Lopez" rel="noreferrer">article</a> and the Chess Opening Theory Wikibook <a href="https://en.wikibooks.org/wiki/Chess_Opening_Theory/1._e4/1...e5/2._Nf3/2...Nc6/3._Bb5/3...a6/4._Ba4" rel="noreferrer">chapter</a> on the Morphy Defense. Lichess' Masters Database has only 132 games where 4...Be7 was played, out of 88,219 (0.15%). What am I missing? Why does no one play Be7 first?</p>
 18809: <p>5...Be7 will almost always transpose into 5...Nf6 followed by 6...Be7, so there's no special line to worry about. However after 5...Nf6 Black still has other options</p>
<p>We usually want to develop first the pieces that we know which squares will go to. The knight goes to f6 on almost every game in this line, but the bihsop can decide later between c5 and e7.</p>
 4418: <blockquote>
<p>Why does no one play Be7 first?</p>
</blockquote>
<p>Because Nf6 is more forcing. It attacks the pawn on e4 forcing white to do something about the threat. Castles indirectly does this by opening the possibility of Re1 if black takes the e4 pawn.</p>
 8860: <p>Practically/psychologically Nf6 makes White's life a bit harder because it also leaves open the possibility of following up with Bc5 (I see that this is called the Møller defense).  This kind of thing is part of why it's a general opening principle to develop Knights before Bishops: usually we know where the Knight goes (here, to f6) while the Bishop has a choice so we keep more options by moving the Knight first.</p>
