Because they can seriously bugger up the display of the pages that show thread titles for those who access the site using certain forms of assistive technology
It's one of the little considerate courtesies of MN that is really appreciated
As is the expectation that people who can write in standard English do so - that's very helpful to the same group (especially the avoidance of txt spk) plus it's helpful to those for whom English is not a first language and for some who live with dyslexia.
Perhaps think of both those things as a form of inclusive action