Deep down they know it all too well, but they don't like the truth, unfortunately.
Thailand is a very dirty and polluted country, which is still very much developing from 'Third-World' status. Cambodia and in particular Phnom Penh is worse!
A quick drive around the back roads of Pattaya will reveal fly tipping everywhere, refuse in large piles outside accommodation blocks, people discarding litter out of their car windows etc.
Again, it comes down to lack of enforcement and punishments for offenders.