I am basing this off of existing frameworks, and anecdotal experience. Take it as you will. I was prompted to think about this in depth by INTJ/INFJ aquantainces that both told me about the rarity of their type within the first minute of discussing MBTI :P
ISTPs and ISFPs are rarer than ISTJs and ISFJs, and no one appears to dispute this at all. This makes sense sociologically and follows the base theory of MBTI and related personality systems. P types are the “innovators”, too many P types in a society would cause it to fall into chaos because they are less aware of the true nature of reality (they have to be or they would lose their idealism and desire to improve.)
The statistics for MBTI distribution are unreliable because the testing methods used to collect the data are not robust (and this is part of why psychologists have outright rejected MBTI as valid... which is pretty sad because MBTI/functional stacks can be so insightful) people idealise intuitive, deep thinking (introversion) and prospecting cognition and unconsciously answer questions in ways that do not reflect their true self, but their idealized version of themselves. Even in people who are honest with themselves, aware of themselves... the questions are still only interpreting behaviour. Behaviour has too many external influences to accurately measure internal cognition. so you end up with a lot of fake INTJ/INFJs but even more fake INTPs and INFPs.
I noticed that INTJs/INFJs seem to identify with their "label" more than any type, want it to feel special/meaningful, and are more aware than other types when someone is mis-categorised as their respective type. They are also seemingly the most VOCAL about these observations. It is a lot harder to be a fake INxJ on the internet than a fake INxP.
INFP/INTPs are more carefree about people labeling themselves, won't really call you out if they think it's inaccurate because they aren't interested in defending their position or enforcing correct labels and aren't as concerned with others being incorrect.
So you end up with a bunch of INFPs/INTPs online that are actually ISTPs/ISFPs. The latter two personality types are the most intuitive S types so it can be hard to differentiate for most. A lot of media online that comes from INTPs/INFPs is not actually reflective of INxPs, and this feeds into the misguided beliefs and understanding people have about these types and makes them seem a lot more prevalent than they actually are.