Because if you clean your gun you will have no problems and get great results.

I have used Moly coated 75 gr HP moly coated Hornady match bullets for several years .
The rifle (AR-15) has had 6-7 thousand of these bullets thru it with no problems.

Where you will have problems is when you fail to clean the barrel every 50 or so rnds.
I then fire 1 fouler.
I read of over 2 thousand rnds thru a barrel without cleaning ,the barrel may have been ruined by the moly build up.