If you've ever had to organise a scientific study you'll understand why. Privacy and ethics are important but they make recruitment of a large and representative sample for any medical study a nightmare. First you'll need to run your protocol past an ethics board and do back and forth until it's agreed. Once you have that done you'll send out an invite (often blind) in hopes of getting people who fit your study aims and you will get who you get. There are often costs associated with this.

So now you have some willing participants you have to screen your initial group to filter it down to people who actually meet your recruiting criteria and consent them for your study. Finally you actually get to gather your data, if you need people to come in for sampling and have to cover their expenses.

Next you'll look at your data and realise there's some confounding effect which reduces your powers to infer anything (e.g. somehow you overrecruited a particular group and they turn out to do something which correlates with the thing you're studying). You'll cry a little and realise you need to recruit more people to have any statistical power to draw a conclusion.

tldr; medical and scientific studies are hard if you want them to actually have any validity.