It's all about the grass. High quality grass requires cold temperatures to grow during the nighttime based on the carbohydrates (starches) generated during the day light hours by photosynthesis. Hence in temperate climates the abundant grass can generate a lot of milk products from genetically selected dairy cattle. There are very few locations in Thailand that can exclusively raise dairy cattle, which require large plots of (hundreds of acres / rai) land devoted to pasture (alfalfa, timothy, clover, trefoil) and hay - which are specialized crops. These do not grow well on land converted from rice fields without a lot of effort to generate these crops sustainably and without specialized farm equipment or irrigation. Salination of those rice fields, after generations of irrigation, further inhibits pasture species.
I find that Thai milk tastes odd - because the grass being fed to cows in my home region of Canada gives the milk a certain taste, that I'm used to; so milk from anywhere else but your home region where you grew up will taste weird. Thai farmers often feed leaves from the "Gatin" tree as they are high in nitrogen and protein - but taste very strong and bitter. You are what you eat - so dairy cows give milk in taste, quality and quantity based on what they eat.
Keep in mind, that traditionally, Thai farmers relied on rice production, not only for their economic livelihood, but also for survival of the family during times of drought, or when flooding killed their other food crops. It is a bold farmer who will give up rice production in favour of growing grass that they cannot personally consume, and that is only used by cattle that are too expensive to kill and eat. Although they never wanted to kill the family water buffalo, they would if the family was starving.
Thai families traditionally give children milk until they are about two years old - but after that, hardly anyone, certainly not adults will drink milk, so the market for dairy milk is quite limited. Shockingly-sweetened milk seems to be an attempt to have people buy milk instead of soda pop and other sweetened drinks.