HA instead of WA in Hiragana

I noticed that many Japanese word ending with the syllable "WA" as "KonnichiWA, KonbanWA" and others were transcribed in Hiragana with the syllable "HA" instead of "WA" that is not the case when the syllable "WA" is at the beginning of the word as Watashi.
Being a beginner I'm curious to have an explanation.
Thanks for your help

The hiragana 「は」 is pronounced as "wa" when it's used as the particle which translates roughly as "As for (the preceding word/phrase)", but is otherwise pronounced as "ha".  To indicate that this is the hiragana being used, when using roumaji, it's still spelled as "ha" even when it's pronounced as "wa".

Those two expressions are shortened forms of an originally longer greeting, for instance, something like "The weather today is good, isn't it?" or  こんにちは、てんきがいいですね。 in hiragana.

Thank you Samuel for this explanation.

Hi there!
 
Keep up the good work and がんばって ください！
Crystal