why is the benoni not played at the top level?

Nonsense. Anyone over 2000 rating knows that the Benoni is passive at worst and unambitious at best. Besides, the Benko Gambit is the more aggressive and more sound version of the Benoni, which explains why it is still often played OTB and online while the Benoni isn't.

Also, the Benko Gambit is known to be one of the most solid gambits in of all chess openings.

Let's not forget that Tal played the Benoni with good results. Then again, over the years, White has figured out ways to put Black on the defensive. Particulary tough is the Taimanov attack which combines Bb5+ with f4 being played before Nf3. I will venture the Benoni only if Nf3 is played before f4. Often my games end up transposing into either a Queen's Indian or Nimzoindian setup.

the BENKO gambit isnt very sound and is often +0.7 for white

the volga gambit is very sound and can lead to very imbalanced postions where whites up a couple pawns but black has a big center

literally the reason people even play the benoni to start with is due to the imbalanced positions, exchange sacrifices, kingside attacks, etc...

you wont find those in many other openings

https://www.chessgames.com/perl/chessgame?gid=1139900

good example game with the benoni, black first targets whites center, after white consolidates it, he targets the queenside, after white stops blacks plans, black strikes with f5, sacs some pieces, and wins the game quickly (in the spirit on the benoni)

Some examples please

just look at most modern super GM games agianst the benoni