Classical Scheveningen: Why is ...Ne8 so rare in this position? Black to move. This position from the classical variation of the Scheveningen occurs over 900 times in the Lichess database, yet ...Ne8 has been played only three times. The move seems strong: it prepares ...Bf6 followed by a trade of dark-squared bishops. Since white's DSB is bearing down on the black king, while black is cramped and his bishop sits apparently without prospects on e7, this trade is surely favorable for black. Indeed, my versions of Stockfish 12 and Leela both give ...Ne8 followed by a timely ...Bf6 as the best lines for black in this position. Leela gives ...Ne8 as the only move that retains approximate equality. Why then is ...Ne8 played so rarely?
[FEN &quot;r4rk1/1bq1bppp/p2ppn2/1p6/3BPP2/P1N3Q1/1PP1B1PP/R4R1K b - - 0 1&quot;]
Haven't checked, but I would guess it is due to a combination of weaker players (the classical Scheveningen is not all that popular nowadays), most games being blitz or bullet and the move not being obvious.
While it might be the strongest move in the position, the maneuver Ne8, Bf6 is not obvious at first glance.  It comes with some downsides which have to be carefully considered before deciding on it:

the move removes one attacker from the pawn on e4 (even though it cannot be currently taken)
the move blocks the rook on f8
it moves an active piece backwards (at least a psychological factor)
white could reply e5?! at one point which is not easy to calculate. It could either be a strong move blocking the knight (and therefore the Rf8) and lead to an attack on the black king. Or it could turn out to be weakening with black freeing himself and moving the knight via c7 to strong light squares in the center. In any case it is not easy to calculate.

Other factors could be the mentioned shorter time control, player strength, or perhaps some players preferring to move the Rf8 before Ne8.