My question is: why is "collecting data" listed as on topic, however, when this question about collecting data is posed, it's put on hold as off-topic?
Your question concerns looking for existing datasets rather than generating data of your own, but please note that
Questions about obtaining particular datasets are off-topic (they are too specialized)
The Beta Open Data site most likely would be interested in fielding such questions.
The sort of things that are on-topic are the statistical aspects of data-collection methods: sampling schemes, surveys, experimental design, &c. So if you want to investigate whether patients with some disease have higher blood pressure than normal, CV would be the right place to ask about choosing an appropriate sample size; but not about how to use a sphygmomanometer, or where to find data on the range of normal blood pressure.