Why do people keep wrecking these boxes?

Why is it, when a drawing-set case is in a thrift-store, the latch of the box is always destroyed?

Probably because nobody remembers these cases. I.e. that these have a little latch at the front-right corner (or sometimes have two, at both front corners). Pulling-out the latch will unlock the box and -voila- the lid can be opened. For anyone encountering one of these baffling boxes; that's how they open.

This was a decent late 1930s to perhaps 1950 set, made by the firm of E.O. Richter in Chemnitz, Germany. A medium-sized set with ink-pen, a large compass, extender and a drop-bow compass. These are pretty common, but are quality instruments; worth trying to fix. 

To repair the latch, an old 2mm knitting-needle was filed down replace the missing section. The combined part (cyanoacrylate) makes the assembled 'rod' have again the slot for the little nail that locks it in orientation and limits its travel in its channel in the wooden base.

After re-fitting the latch (slide in from the right), placing the locking-nail (through the slot and push firmly into the wood), the purple box-lining is carefully glued down again as is thin paper-edging. An almost invisible repair, this.

These larger Richter-type compasses have a little mechanism that keep the handle straight - this must not be forced; you can't open the legs by pulling one leg and holding the handle. If you do, then the small guiding-rod snaps. That's probably another bit of once-common knowledge that's no longer all that common: the compass should be opened by holding the legs, not the handle. Thrift-store: so a broken guiding-rod. This little guiding-rod is now replaced by a new 'rod' fashioned from a length of M1 threaded rod and two (relatively massive) M1 nuts. (In picture below, centre, they're really rather small :-)

Yet another thing with thrift-store drawing-sets is that bits go missing. The case gets opened (broken open), handled and something will accidentally drop out and is lost amongst the rest of the jumble. This set is only missing its needle-attachment for the large compass. I have a spare part for this, but no needle-screws yet, so this is something to hunt for (or re-create).

These fairly-common Richter sets are difficult to date reliably, but the black tapered plastic handles would make it not earlier than the mid 1930s. The old style of Richter-brand and absence of any GDR quality marks suggest it is earlier than 1950 and probably before 1945.

In any case, the latch-repair turned out well and even though these sets are common (and not expensive), it is a very nice and still usable drawing-set. (And at a fiver, no regrets about getting it :)