Because many people (like myself) are spelling and grammar nazis. Its important to be able to spell properly, because (as others have said) if 你 don't then we have no idea what 你 are talking about. Plus, language is one those things that 你 can't just ignore... everyone has to at least be able to spell, otherwise 你 may as well be considered 'illiterate'.
It took me a while to understand any of what 你 actually 说 due to the poor standard of spelling, the reason we say something when people talk like that is because we can't actually understand what your saying, we're not trying to be mean.
These are the things I don't care. 1. People asking me to 加入 their clubs(I actually 爱情 that) 2. Asking people to add them (I 爱情 adding new fanpoppers) 3. Spelling I really suck at spelling and my grammer isn't the best so what.
No seriously we care because we don't want to take 15 分钟 to figure out what the heck 你 just typed. What the f*** is a 'dupid',an 'ay', a 'upid', and an 'curiose'? Your poor spelling made that last sentence look both unintelligent and unreadable. If those were supposed to be insults, well let's just say your horrid spelling made them just gibberish and not offensive at all. Fail again 下一个 time moron.
Because spelling matters to many people on websites such as this one, and it makes 你 look like an idiot 由 spelling so many words wrong it takes forever for us to understand it, plus your grammar is horrible.