Because injury will set you back far more than missing out on a few reps.
"To failure" is a relative term. Someone who has lifting for a very long time knows their body very well. "To failure" some days might mean they don't even finish their warmup.
Funny you should say that as I remember it coming up in the counterarguments at the time. Both in terms of neuromuscular control and also general personal motivation type angles.
If you can still maintain form while it's hard, then you haven't reached the limit yet. Velocity of a lift decreases as you approach failure (you're in the "hard zone"), but you should keep going until you either can't complete rep without twisting and cheating, or if you know yourself well enough that you know you'll have to do that on the next rep. Periodically you should try going to actual failure to make sure you know what that really feels like.
That seems funny to me. While someone's form breaking down leads to inefficiencies in specific muscle targeting and / or an increased chance of injury I think that approach would end up leading to people bailing when things start to get hard. I've seen a lot of people never put additional weight on the bar because they were afraid of their form breaking down when in reality measured incremental load increases are what helps makes one big and strong.