no doubt, but how do you possibly handle things like that? if someone blatantly knocks-on and the ref misses it, but the play goes for another 15 seconds, when and how do you put it up for review? if the attacking team is likely to score on the next tackle because they made a break and have a 5 man overlap you cant call it up and have a review then. if someone knocks it on, they call play on, then he gets tackled straight away and is in his own 20 then sure it wouldnt have an impact to look at it. but who decides when it would have a negative impact?
thats why its better to just be able to challenge calls that the referee has made, as there is always a stoppage of play. if a referee awards a try but you think they knocked on in the lead up, challenge it and if they did it will be reversed to a no-try and a knock-on.
theres just no good or easy way to officiate on non-calls unfortunately.