I don't write like I talk, because my speech has a bunch of filler in it. Moreover I am often talking at the same time as forming an opinion. (ie trying to figure out why something is the way it is, by asking questions.)

I flatter myself into thinking that I am a eloquent and concise speaker, having listened to my self talk, I can say thats not the case.

The joy (and curse) of writing is that you can condense everything down into nice, tight paragraphs. You can re-order arguments in a way that doesn't make sense orally.

If a rich person tells you to write like you talk, its because they either have the privilege of a journalist editing their quotes and stringing them together in a way that makes sense, or people read what ever mountains of waffle they produce because they are rich.

The point of writing is to get your point across in as fewer words as possible, the point of talking is to socially interact, they only sometimes align well.