Because 'badness' of the obvious kind is an easy way to create drama. There are lots of cliches like that on TV.
How many series have there been featuring the 'maverick' loner detective whose marriage broke up because he was so devoted to solving cases he was never home, and who forgets to pick up his cute daughter (almost always a daughter)?
How many US police dramas feature a shouty black lieutenant or captain who is constantly chucking files, bellowing at our heroes and complaining he is 'too old for this shit'?
Every single programme, good or bad, featuring lawyers pursues the maverick, anti-establishment lone crusader drinks red wine late into the night over mountains of case papers angle, the cliche being the default position of lawyers is to be creepy, manipulative agents of the elite. True or not, it does get boring as drama.
Teachers are either evil shouting bullies or saintly inspiring figures with a vocation to reach even the most off-putting children.
I reckon that is why people liked Taggart so much when Marl McManus was in it. Forget the cliches, he was a ruthless, abrupt, granite-faced wee bastard, which came as a welcome break from all the jazz and whisky-loving, scruffy yet inexplicably attractive to women bollocks screen-writers usually come up with.