There's a couple things at play here.
Firstly, the math. Nutrient requirements in a given population tend to be normally distributed (there are a couple that aren't). RDA is considered enough to meet the requirements of 98% of people. EAR (estimated average requirement) is the middle of the bell curve, so most people's actual requirement will be closer to that. RDAs are used for diet planning in individuals though, so as to minimise the risk of deficiency. There's a huge margin of safety between EAR and anything that would cause problems from excess (e.g. Vitamin A toxicity) so using RDA won't cause issues there. Actually working out an individual person's requirement for a specific nutrient is a bunch of really complicated biochem, so it's easier and safer to just aim for the RDA in that context.
Secondly, RDA is kind of a misnomer. You don't actually need to eat that every day, because the body stores and uses a lot of nutrients as needed. It's really supposed to be used as more of an average over time. This is why it's important to eat a good variety of different foods within the main food groups. Meatless mondays don't cause you to instantly collapse from iron deficiency.
Third, there's a commercial aspect to it. Food manufacturers love using RDA because it lets them imply their product is healthy, and they love it when public health messaging is "eat more X" when X is a major component of whatever they produce, or something that's cheap to fortify (e.g. iron fortified cereals). This way they can encourage people to freak out over "oh no, I need this product or I won't hit my RDA!" Really, most people better off eating generally healthy (roughly 50% fruit and vegetables, 25% protein and 25% whole grains) with a good variety of foods than aiming to hit the RDA of everything, every day. There are a few caveats to that for certain nutrients in some circumstances, but on the whole, no. You don't need to hit your RDA every day, but it probably should average out over a week or so.