Because in a recipe it's impossible to specify cooking times without pre-heating. It's easy to say 10 minutes at 200 degrees, because this would be exactly the same for everyone. Every oven is different, so the time would be different depending on your oven, which the person writing the recipe can't know. So if the instructions on something like bread say pre-heat and bake for 10 minutes at 200 degrees, they know the result would be good.

There is also the fact ovens warm up differently. If there is a heating element within the compartment where the food is being heated (especially above), this element gets way too hot and emits a lot of infrared radiation whilst heating up the oven. It does this because it wants to get to the set temperature as fast as possible. Once it gets there it only needs to maintain that temperature, which is much easier, so the heating element gets much less hot during this time. If you set something like a cake in the oven with a heating element right above it, best case the top of the cake gets baked more than the rest, worst case the top gets burnt before the inside cooks.

Then there's the fact whilst heating the temperatures inside the oven fluctuate a lot, some parts get hot fast, other parts take more time. When you have food that's sensitive to that you def need to preheat.

And there's a lot of chemistry going on, for example some foods get really greasy if they don't get hot enough while cooking. Whilst these food could be cooked with the temperature going from 50 - 150 degrees, the end result would be much better if it's just cooked at 200 during the whole process.

Now there are a lot of cases where this doesn't matter and if you know your oven well enough you can compensate. But there are plenty of legit reasons to pre-heat and you may even have better results when pre-heating, even if the end result was fine before.

So I agree, people should pre-heat and there are tangible benefits!