Short answer... yes... tents and camping equipment SOMETIMES gets stolen from campsites, but MOSTLY it doesn't. If it was the other way round, campsites would be closed down within a week! It all depends on who else is mooching around the site, and how desirable your kit is. Even when no-one nicks your stuff, it doesn't stop other problems. I once watched a cow in Langdale (Bayesbrown Farm) get tangled in the guyropes of a family tent, and rip it out of the ground once panic set in!
A story from my youth... a wild camp consisting of 20 tents was said to have been lifted from Angle Tarn while the campers were on Bowfell, along with all the best bits of kit. When things happen on that scale, it involves a lot of thieves!
Crikey... I'm making Langdale sound like a no-camp area!
My preferred approach to camping in general is to pitch my tent and put my head down for the night, then pack away in the morning and move on. Less UV radiation hitting my tent, and my backpacking kit doesn't weigh a whole lot more than a day pack anyway.