Like all the other answers, this is just going to be a guess, but this one is backed up by actual history.
It's because of how Steam packages files.
Valve's games made prior to Left 4 Dead all include the Half-Life 2 Shared Files.
One of my old answers breaks down how the GCF files were laid out for TF2 specifically.
There are 2,722.83MB taken up by shared files, including the
multiplayer ob binaries.gcf file, which would include the version of
hl2.exe used by all of Valve's older multiplayer titles.
Note that there was likely also a single player binaries gcf file used by HL2, both of its episodes, and Portal 1.
These GCF files were shared between games.
In 2013, Valve switched over to the VPK system. This renders most of what is listed here as moot as the HL2 "Shared Files" are now distributed as part of the game itself. Valve likely keeps the executable with the old name out of laziness.
Incidentally, in games from 2008 or later, you will likely see the executable named some variation of the game name since they didn't use the shared files.