You're asking the question the wrong way round. You should be asking "Why do people spell こんにちは konichiwa rather than konnichiwa?" and the answer is "because unfortunately they don't seem to know any better"...
– EarthliŋMay 30 '18 at 18:56
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It's actually konnichiwa(こんにちは). We don't say konichiwa(こにちは).