user14202: After
[FEN ""]

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nd4 Nf6 5. Nc3 a6 6. Be3 e5 7. Nf3

(which is trendy at top level in 2017) I am wondering why Black doesn't play 7...Ng4.   I only found a couple old games with 7...Ng4, and both ended in a draw.

user14079: 6...Ng4 doesn't look good at all.
Tactical flaw:
Black is behind in development, so White should aim for direct play with 7.Bg5 and then:

7...f6 8.Bc1 [8.Bh4] Be6 9.h3 Nh6 10.Bxh6 gh6 11.Nh4 [11.Be2] and Black has huge weaknesses on the light squares. 
7...Be7 8.Bxe7 Qxe7 9.Nd5 [9.Qd2, 9.Bc4] Qd8 10.Bc4 with a nice bind is probably the lesser evil for Black.
7...Qb6 8.Qd2 [or 8.Bh4 Qxb2? 9.Nd5 and Black cannot block Nc7+] Qxb2 is too greedy: 9.Rb1 Qa3 10.Nd5 Qc5 11.h3, or 11.Be3 and 12.Nb6 winning material, or 11.Rb3 with a huge attack.

Strategic reason:
It the structure with a hole on d5, the fight for the white square is essential and probably more important than the bishop pair. White often plays Bg5xf6 in Boleslavsky, Najdorf and Svechnikov variations. Thus trading the knight for the Be3 is not a big achievement, especiallly at the cost of two tempi, even if White plays the simple 7.Qd2 Nxe3 8.Qxe3 Be6 9.0-0-0 or 9.Ng5.

14202: <p>After</p>

<pre><code>[FEN ""]

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nd4 Nf6 5. Nc3 a6 6. Be3 e5 7. Nf3
</code></pre>

<p>(which is trendy at top level in 2017) I am wondering why Black doesn't play 7...Ng4.   I only found a couple old games with 7...Ng4, and both ended in a draw.</p>
 14079: <p>6...Ng4 doesn't look good at all.</p>

<p><strong>Tactical flaw:</strong></p>

<p>Black is behind in development, so White should aim for direct play with 7.Bg5 and then:</p>

<ul>
<li><p>7...f6 8.Bc1 [8.Bh4] Be6 9.h3 Nh6 10.Bxh6 gh6 11.Nh4 [11.Be2] and Black has huge weaknesses on the light squares. </p></li>
<li><p>7...Be7 8.Bxe7 Qxe7 9.Nd5 [9.Qd2, 9.Bc4] Qd8 10.Bc4 with a nice bind is probably the lesser evil for Black.</p></li>
<li><p>7...Qb6 8.Qd2 [or 8.Bh4 Qxb2? 9.Nd5 and Black cannot block Nc7+] Qxb2 is too greedy: 9.Rb1 Qa3 10.Nd5 Qc5 11.h3, or 11.Be3 and 12.Nb6 winning material, or 11.Rb3 with a huge attack.</p></li>
</ul>

<p><strong>Strategic reason:</strong></p>

<p>It the structure with a hole on d5, the fight for the white square is essential and probably more important than the bishop pair. White often plays Bg5xf6 in Boleslavsky, Najdorf and Svechnikov variations. Thus trading the knight for the Be3 is not a big achievement, especiallly at the cost of two tempi, even if White plays the simple 7.Qd2 Nxe3 8.Qxe3 Be6 9.0-0-0 or 9.Ng5.</p>
