Why do you believe some SMs feel the need to embellish the level of their service by lying about their MOS or deployment experience?

I'm glad I got back alive. Who the hell wants to talk about it. Just keep the fireworks away.

I spent 5 years, 9 months in relative boredom. And i am okay with that

There is only thing I embellish / lie about and that's the size of my ...
Hint "it's HUGE"!

Oh,, I got lucky and had some vic's EID'd and then shot. So I only lie a little bit about having a hot Iraqi mistress I would visit in Karma during our patrols... Of course I could never tell which one she was..

The vast majority of "posers" are just blowhards trying to impress friends, co-workers or strangers at a bar. Most people see through their bullshit just like they see through people who claim they were high caliber athletes or have a Porsche that is "always in the shop".
Generally those are the type you just walk away from when they start yammering and if they CONSTANTLY keep it up just tell them they are full of shit and leave it.
I peg the needle when someone makes up stuff to get government benefits, discounts, or scam charities. It also depends on what platform they use. A bar stool hero talking to 3 or 4 people isn't the same as some dirt bag who dupes a newspaper or TV news station into publishing their tale tails. Anyone who lies in the media should be called out in the media. Anyone who gains tangible benefits should be jailed for fraud.