It's a way of making sure the same words aren't used multiple times in the same sentence. Don't use more words than you have to!

"but" is linking to something before (even if it was a different sentence - a common error with "but"), so they have already used "I" and didn't want to repeat it again. For example: I'm not explaining it completely but (I) am covering the main points. He is not here but am, [NOT OK - because 'he' and 'I' are different] I like it but (I) am just thinking about whether to buy it or not. [OK - even though 'like' and 'am' are different]

I met a lot of people using 'Am'. And they are all the scammers from Nigeria.. so be careful when someone texting to you like that.

It should be "I'm" instead of "am" as it is the correct abbreviation for "I am". "Am" is not proper English.

Matt described it perfectly. You can even drop the second 'am'. "I am not trying..., but pointing..." or "I am not trying..., but I am pointing...".