Why is it so hard to get legitimate parasite testing?

how inaccurate are the general ones most hospitals do?

aren't parasites a major concern for travelers, especially those who get sick while abroad and whose symptoms persist even after returning? this is my situation. I was in China, so I think it's reasonable to suspect a parasite and my symtoms started immediately after my sickness in China stopped (I went there, ate some bad food/water or something, had diarrea for a little while, then my digestive system has never returned to normal, constipation/bloating, which i've never had before in my life. I'm 19 years old)

I had the simple stool test done by the regular hospital I go to, and they found nothing, which I've read is common of those tests.

But surely this must be a pretty big thing amont travelers, right? Am I supposed to find a tropical medicine department or something?
It's been driving me crazy and my skin went from perfect to covered severely in acne (which I believe is directly related to whatever is going on, either the constipation itself, or the pathogen or something). My parents didn't listen to me when I said something is really wrong with my gut until they saw my face undergo this drastic transformation. My face is completely scarred, and finally they're realizing something is wrong.

I feel like it's a simple parasite problem given the circumstance, and it blows my mind why they don't have accurate tests for these. I guess I could go ahead and buy an antiparasitical drug, but I don't really know the protocols or anything.

So, I'e made an appt with an alternative medicine doctor who can order a private lab stool test for me, however all this costs a lot of money and I'm just doing it to cover my bases I guess in case it's not something as concrete as a parasite that I need to get rid of. But I really don't feel like it should be anything complex. I'm 19, have eaten healthily on the whole these 19 years, am active, have never had constipation problems in my life, so I don't think any bad lifestyle habits have accumulated yet to the point of really affecting my gut so drastically and suddenly

You might want to check out "Humaworm"