Why is the Rapture not mentioned in the Old
Testament?

The prophets of the Old Testament were not
given revelation of the Church, the Body of
Christ.

They were not told of the Rapture of the Church.
This a New Testament revelation. The Bible
speaks of the message of salvation by grace that
was given to the Church to give to the world.
1 Peter 1:10-12
"Of which salvation the prophets have enquired
and searched diligently, who prophesied of the
grace that should come unto you:
Searching what, or what manner of time the
Spirit of Christ which was in them did signify,
when it testified beforehand the sufferings of
Christ, and the glory that should follow.
Unto whom it was revealed, that not unto
themselves, but unto us they did minister the
things, which are now reported unto you by
them that have preached the gospel unto you
with the Holy Ghost sent down from heaven;
which things the angels desire to look into."