The most common and most likely answer to this crossword clue is the 7 plus 10 letter word BECAUSE IT’S THERE
I've seen this crossword clue in The Guardian
B
E
C
A
U
S
E
I
T
â
€
™
S
Â
T
H
E
R
E
Buddy explains!
The straight part
We know that "Why climb Everest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case why climb everest = because it’s there.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.