The slightly more charitable answer is that having your infrastructure automatically scale and adapt to the workload, and watching it take off is a really fun and exciting moment, and people want to experience that, and in the rare event that it does happen and you suddenly need to handle 10-100x more traffic, having things auto-scale up and then back down means you (in an ideal world) get to watch it excitedly, instead of furiously running around spinning things up and putting out fires that arise when you start operating at larger scales.

Is it overkill to build your business like that from the get-go? Almost certainly (precluding the situation where you have some kind of guarantee of incoming load), but people (want to) do it anyway.