A quote, apparently falsely attributed to bank robber Willie Sutton, but still apropos vis-a-vis checked luggage, is "Because that's where the money is".
Bags are broken into in airports:
Your Checked Bags Aren't As Safe As You Think They Are
....and that's in the US alone AFAIK.
If, and it seems that they are, bags are broken into, which ones do you think are the targets? Yes, me too...."Because that's where the money is". Can't find stats on the average cost of the bags generally stolen from, but I'm willing to bet it ain't the grungy ones.......(as I'm fond of saying "Our bags are so beat up that people open them and put stuff in")
We're trying carry on backpacks only for our upcoming trip.
I read a book by Willie Sutton titled Where The Money Was, so I think that settles the attribution question.