Emerson is on the right track. I think the main answer is:

Because society can’t handle an INTP. Most of society is built on structure. Both physically and mentally. Things are taught a certain way, things are communicated a certain way, and things are built a certain way. INTPs generally don’t (can’t, actually) follow that structure naturally.

As such, society generally tends to retaliate against an INTP from an early age. Whether it is ignoring their incessant questions (why?), not adjusting the school curriculum to meet their needs, or determining their solutions and ideas are crazy or untenable, society teaches an INTP from an early age that they don’t belong, that they won’t be taken seriously, and that they are broken/flawed.

Add to this an INTP’s lack of communication, social skills, and lack of outward organization, and the problem is amplified.

Thank goodness it’s a rare type, so those impacted by society are few.