I understand back in 2013 when gog galaxy wasn't around but now that it is available, it works similarly to steam's game launcher, so why isn't there a patch yet to add in the daily challenge as an option?

Because the dev moved on and active updates for the game ceased after a year or three.
Who knows, maybe this version will get an update to the match the Steam version if Spelunky 2 ever comes here, but I won't hold my breath. (Dailies would almost certainly need to be played through Galaxy anyway, which would still leave out those of us who just don't want to use a store client at all.)

at least galaxy is more lightweight than the steam client. you can just get a shortcut from the game folder to play without launching galaxy, i do that all the time.

Something something bothering with the effort of a server.