Its cost, losing kidney function and the subsequent dialysis is more expensive than a 90day check up. Besides you dont want to lose kidney function if all it took to avoid it was a 90day check.
Saying that kidney function usually involves Estimated Glomular Filtration Rate which means they measure the creatinine. Now if you happen to supplement with creatine, creatinine being the metabolised form will alter the eGFR! So if a dr doesnt ask you if you are supplementing with vitamins, minerals and other supplements, you could skew some medical tests and end up with a medical condition that doesnt exist!
You can also get quite alot off your pharmacy, but you need to know what to say, so some online pharmacys are better to use to work out what to say.
For example, if you want to get hold of Testogel here in the UK, for males its a nice pick me up and its being used in South Africa for dementia treatment, you need to say your T levels are below a certain amount. If you get rejected, you lower the amount on another website and then you can eventually work out what the level is that lets you have a testogel prescription!
Its not too hard to game the medical system for low risk drugs, for everything else you have various people who already qualify and through word of mouth have been known to sell on their meds because the Dr might sometimes up the dose if there is no response or told there is no response. SSRI's are like this. In theory a blood test measuring the 5-Hydroxyindoleacetic acid from the jugular vein will test if someone is taking SSRI's but they also do other things which go undocumented until some published scientific study highlights it.
To be honest though, the best drugs are vitamins and supplements, the number of medical studies I've read highlighting negative effects is quite shocking. Take https://en.wikipedia.org/wiki/Bisphosphonate#History
It was first used for bone problems in the 1960's but the technology wasnt around to explain exactly what it was doing until the 1990's! Loads of studies slate the use of Bisphonates but these are recent like post millennium studies.
In fact you go through Google Scholar, you can see a trend from the first studies published in the 1800's where medicine was largely using what was found in the body, so B12 deficiency was eating raw liver, then from WW1 to WW2 patented drugs started to appear and thats when modern medicine started, but it is a case of medicine is restricted by the technology so AI in medicine is big business.
This gives you an idea of how the medical profession work, drug companies develop something run a battery of tests and then see if they can get it licenced and sold as a treatment.
https://en.wikipedia.org/wiki/Bupropion#History
This is another drug designed for one thing but they noticed it had smoking cessation properties so it got relicensed.
Painkillers even the common ones like ibuprofen and paracetamol are also dangerous drugs. Paracetamol will destroy your bones and your kidneys dont like ibuprofen.
This is the history of Ecstasy aka molly aka mdma
TLDR,
1912 German pharma Merck developed the compound for blood clotting, didnt work so got shelved.
1950's US Army dust off the compound and get university of Michigan to study it.
1973 results declassified.
1978 https://twitter.com/DrAShulgin a former Dow chemicals chemist into his psychoactive drugs looks at it and publishes a report
"the drug appears to evoke an easily controlled altered state of consciousness with emotional and sensual overtones. It can be compared in its effects to marijuana, to psilocybin devoid of the hallucinatory component, or to low levels of MDA"
Psychotherapists start using it.
1980's its getting popular and DEA start to take notice.
1985 DEA get it banned, people having too much fun!
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3931692/
Medicine is like chemical hacking of the body.