It's puzzling, at first, but when you understand the culture and its flaws and consider the way some of these *ahem* 'men' are raised, then I would say it becomes clearer and easier to comprehend.
Too many Thai males are not raised by their parents, but instead by an extended familial unit, or more distant family members, very often it's the grandparents, either through inability or unwillingness of the parents to do the job properly (for a multitude of reasons) or because they are absent, often due to them working in another area of the country where more profitable employment can be obtained. There's nothing inherently 'wrong' with this situation, plenty raised in such circumstances will turn out normal and decent, but that is usually predicated on a strong and functional family unit. However, there are definitely many (males particularly) who don't receive proper discipline and are as a result sorely lacking in respect for others and/or able to differentiate between what is socially acceptable behaviour and what isn't. Being reared (often) by aging and obviously less energetic, or engaged family members as well as the tendency for dysfunctionality in such situations has to be responsible for a lot of the bad character traits we're discussing here, IMHO.
The lack of a male disciplinarian in the form of a father-figure is sorely missing from many of these young men's lives, this is something that can lead to a whole host of societal issues - as we have witnessed in the West with the state-sponsored breakdown of marriage and the two parent family. It is often exacerbated in Thailand by the mollycoddling of boys by an abundance of female family members and a lack of male family members, the inclination by many (something I have witnessed) is to give boys more leeway and if they act up, no to discipline them sufficiently - possibly a cultural phenomena more uniquely Asian - at least in modern times. As these boys get older and become more physically imposing and thus even less easy to control, the situation simply becomes worse and the poor behaviour more entrenched. This lack of respect stemming from an absence of discipline and perhaps too much attention/pampering as children leads to grown men with the most fragile of egos and a superiority complex - an awful combination. They aren't used to being told 'no' or not getting their own way. Combine that with an all too prevalent, xenophobic and nationalistic streak and of course the famous inability to take criticism or lose face (practically a Thai cottage industry) and the results are predictably woeful, especially with incidents involving foreigners. I will say now that I've never had a big problem with Thai taxi drivers, I'm a resonably young and athletic man and can speak Thai pretty well - I always try to be affable, but I am also firm if I feel that I am being taken for a ride. Probably largely due to these aforementioned factors, I have been lucky not to experience too many issues in the 10 years I've been here.
I have many friends and acquaintances (especially those who don't speak Thai) who have not been so lucky.