Although it's difficult to show a formal reasoning, it could be said that reducing pointless kanji usage is undeniably an orthographic trend of post-WWII era. "Pointless" roughly means a word no longer preserves the meaning the kanji which assigned to it suggests, or in today's linguistic jargon "semantically bleached".
Things like 補助動詞, including ~てください, are typical examples of semantically bleached words. They're very similar to some English words such as be going to, which now simply indicates time progress without actually going anywhere, and accordingly lost its original spelling becoming gonna. There was a government directive issued in 1981 (and updated in 2010) explicitly named ください and a lot of other words need to be written in kana in official documents.
Greeting clichés are also used without thinking of the original meanings. In English we can say goodbye without fancying how a bad bye would be like (well, etymologically it's from "god be with you"...). Your examples おはようございます, こんにちは, ありがとう seem to be retaining their full form, which in fact not, because:
Expected pronunciations: おはよう{LHHL}ございます{LHHHL}, こんにちは{HLLLL}, ありがとう{LHHHL}
Actual pronunciations: おはようございます{LHHHHHHHL}, こんにちは{LHHHH}, ありがとう{LHLLL}
So it's clear that people no longer take them as a meaningful words they look like. The fact many people spell こんにちわ and こんばんわ instead of は also proves the origin of them has totally forgotten.
Of course these are merely "guideline" and not rigid "rule" so you have no obligation to follow, but it's also true that those conventions are steadily maintained across official, media and pedagogical writings.
御早う御座います・今日は・有難う御座います
But it's usually recommended that you write these words in kana.