You could just as well turn the question around: What's the rationale for bothering with several sets of knifes and forks? Minutes traces of food surely aren't a big problem and it does involve quite a lot of work, not only for cleaning but also for the service staff. One way or the other, such things are necessarily customary and this only strikes you as peculiar because you are used to something else.
That said, most restaurants in France will in fact provide a clean knife and fork and fancy restaurants will have several sets of utensils and a plate already on the table when you arrive. You start with the outermost set and then move on to the next one. The cutlery for the main dish are “stand-ins”, the staff will replace them with the correct ones depending on what you ordered shortly before bringing the food. Similarly the plate will be removed and replaced by the actual plate containing the food.
Simpler restaurants will have no plate and a single set of knife/fork and only bring special utensils like a meat knife as needed (possibly even putting it on the plate itself, e.g. with the blade under a steak to hold it in place). I have seen that in cheap café-restaurants, possibly some brasseries or chain restaurants. It does not bother me but I would not say it's common in France.