Because there can be no guarantees with sexual health, in the same way a condom is not 100% guaranteed, nor the pill 100% guaranteed.
If you follow your logic that there are no cuts or brasions in the mouth but are in the throat then swallowing a load would pass by them, but if you expand that further as the girl is sucking said organ then precum and other juices will be swallowed during the owo, and after she has spat the spunk there will be residue in her mouth and this will be swallowed with saliva.
I doubt it would be possible to provide OWO to completion and then spit and not have residue of semen / fluid contact your throat.
Would you like to try some unscientific tests?
Think of it this way,
If you lay on a Lilo in the sea for 24 hours you may get hit by a ship,
If you lay on a Lilo on a Motorway for 24 hours you may get hit by a car,
Ok both statements are true, however I would wager that in reality you would not get hit by a ship, but by the same token at night on a lilo on a motorway I'd say very likely.
BUT you can't say either as a fact as you could be unlucky wiht a ship, or your lilo could be missed by the cars, that's why the word May is included.