I recall being told in the dim and distant past, I think by a security officer in a UK airport, that the way some scanners highlight nefarious liquids relies on the bag having an airtight seal which allows vapour/fumes to build up inside the bag to a detectable level. The zip-type bags don't achieve this.

This could, of course, have been the biggest load of baloney you've ever heard, but there is a degree of plausibility to it - albeit that would mean a different technology was in place for scanning checked luggage.

With the next generation of scanners, which all bar a few tiny Scottish island airports have to install by June 2024, all this should go away because everything will stay in our bags anyway.