Excellent - try this,
1 - quit vacuuming your synthetic carpet and start using soap, water, and drying towels to remove the deep dirt that's in the high traffic areas
2 - Next, try it with just a vacuum cleaner, no soap, water, solution, etc. and see if you did indeed get all that deep dirt out of there with method "1" - we can take bets on if method "1" got it all.
3 - Third, go rent a commercial carpet cleaner which will apply a cleaning solution to the carpet, then vacuum it up and see if method "2" managed to remove all the dirt - again I'm open to bets on if methods "1" and "2" combined got it all
It's the same deal with records - stuff gets buried in those grooves and the RCM insures it all comes up. I can hand clean a really dirty used record 5 times and still get gunk on my stylus; or vacuum my carpets every day and still get disgusting amounts of gunk when I use a professional carpet cleaner. I use my RCM one time and it's as clean as it can possibly get.
That, my friend, is the reason I have hardwood floors and a VPI 16.5 RCM!