That's a great question. Typically は is pronounced as ha, but in this word it is pronounced as wa because it in this word and words like it, は is used as a topic marker. I saw a Japanesepod 101 video that talked about the history of the words こんにちは and こんぼんは. Essentially people used to great each other by saying "Today is..." or "This evening is..." with each followed by the weather. I'll have to find the video and post it here.
I almost reported it as an error until I looked it up and found this explanation here: https://www.thoughtco.com/japanese-greetings-2028140. Konnichiwa is a basic phrase, but in this stage of lessons the primary function is memorizing hiragana, and being introduced to a word that contradicts what we're being taught is confusing. With the cards that introduce you to は the voice also pronounces it as wa, with no explanation. Again, I thought it was an error and reported it. Without having a lesson on particles yet, or at least a quick side note with the mention of exploring it further later on, it just feels like a bad way to learn. I find myself trying to "speak over" the voice in the lesson cards with my inner voice so I can memorize the ha sound properly and not confuse it with わ. My feeling is that the voice for the は cards should be updated to the "ha" sound and こんにちは should be moved to a later lesson for when it's appropriate to introduce the grammar rule of particles that affect は, わ, and others.
You use it as wa very very frequently, seems like nearly every sentence actually since it is the topic particle, it would be useless to not put that in early, it should just say both.
But as far as why, it's no different than in english, sometimes a letter can be pronounced differently, long or short vowels, words like knife, with the silent k, or even an entire word, like read and read, past and present tense, both pronounced different.
It's absolutely necessary to know it as both, from the start though. Because you can't say basic phrases like my name is without using は pronounced as wa.
Just shrug and whenever you find something that is odd like that try and remind yourself of the odd things about your own language (like the word language, lol) which we are used to and take for granted as just the way it is. Imagine learning how to say learning, where the e and a dont make the "correct" sound. Eventually this will be just as natural, if you stick with it.
Languages are weird, nuff said, lol.
'lan-gw-ij' is how it is said. but if you tried to read language phonetically it would sound kinda like 'lan-gu-ag-e' but we are so used to reading it, so we don't think anything about it.
English is one of the least phonetic languages, and Japanese is actually one of the easiest to read once you know all the characters, the hard thing is a handful of odd outliers like this, but once you know them, you're set.
That's right. I remember how much trouble i had in school learning English, trying to understand why are some words pronounced different although they are written similarly, why are other pronounced same etc. It was confusing in the beginning, especially since my language is the only completely phonetic language in the world. After some time i adapted and it wasn't so difficult anymore. Until i started learning French in high school
今日は: 今 = kon, meaning this / now / current, and 日= nichi, meaning day (also used for weekdays etc.). は is the particle "ha" that reads like "wa" when it's at the end of the subject. It literally means "today is" and it used to be followed by stuff like "today is nice / sunny / windy" etc. It became a greeting so the literal meaning was kinda lost, just like the need to follow it up with the weather. Now it just means "good morning", much like "bless you" was once "god bless you" and now it's just a way to wish someone who sneezes to get better.
From the top answer, http://gogen-allguide.com/ko/konbanwa.html こんばんは is correct, according to that page in Japanese.
My gut feeling is the same - 今晩は -> こんばんわ.
That said, a cursory Google of こんばんは yielded 13M hits, whereas こんばんわ yielded 26M.
わ is used so frequently that even some native Japanese use わ instead of は.