Titans seem to be a marvelous piece of technology from the past, so advanced that they can no longer produce them. So the Imperium oft launches whole campaigns, sacrificing a lot of lives and resources just to get hold of a single Titan.
What makes them so valuable? As far as I have seen it, they don't use them to try to understand the technology involved, they acquire them only to be used in battle. However, the Imperium possesses equipment with much higher firepower, and a titan is a large target, and can be wrecked with a well placed hit from above, nuked from orbit, etc. As it is a lone target, maybe the only one on the whole planet, the opponents would surely target it from orbit as highest priority.
Maybe when they could be easily mass-produced it was worth it, but now, are their military capabilities really worth the time and effort to acquire them?