Sure, but cost/benefit analysis says it's not worth the time and effort. The rank insignia have tradition behind them that the military wants to maintain.
It's HARD to fake the paperwork. And that's what you'd really need to fake it effectively. People will catch you very quickly.
I also don't think there's enough benefit to anyone faking it for it to matter either. People do "stolen valor" all the time, and what do they actually get out of it?
So what's the motivation to fake it in the first place?