It's hotly debated, and the literature is often in conflict as to the exact cause of simulator sickness. Regardless of the exact reason, sickness of roughly half the population appears to be somehow fundamental to VR.

Here's some interesting reading:

https://en.wikipedia.org/wiki/Simulator_sickness

https://news.ycombinator.com/item?id=5265985

> In a study conducted by U.S. Army Research Institute for the Behavioral and Social Sciences in a report published May 1995 titled "Technical Report 1027 - Simulator Sickness in Virtual Environments", out of 742 pilot exposures from 11 military flight simulators, "approximately half of the pilots (334) reported post-effects of some kind: 250 (34%) reported that symptoms dissipated in less than 1 hour, 44 (6%) reported that symptoms lasted longer than 4 hours, and 28 (4%) reported that symptoms lasted longer than 6 hours. There were also 4 (1%) reported cases of spontaneously occurring flashbacks."

Simulator Sickness in Virtual Environments (PDF): http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA295861

Some interesting quotes from the report:

Expected incidence and severity of simulator sickness in virtual
environments

> In an analysis of data from 10 U.S. Navy and Marine Corps
flight simulators, Kennedy, Lilienthal, Berbaum, Baltzley, and
McCauley (1989) found that approximately 20% to 40% of military
pilots indicated at least one symptom following simulator
exposure. McCauley and Sharkey (1992) pointed out that pilots
tend to be less susceptible to motion sickness than the general
population due to a self-selection process based on their
resistance to motion sickness. Since VE technologies will be
aimed at a more general population, such selection against
sickness may not occur. Thus, McCauley and Sharkey suggested
that sickness may be more common in virtual environments than in
simulators.

Findings

> Although there is debate as to the exact cause or causes of
simulator sickness, a primary suspected cause is inconsistent
information about body orientation and motion received by the
different senses, known as the cue conflict theory. For example,
the visual system may perceive that the body is moving rapidly,
while the vestibular system perceives that the body is
stationary. Inconsistent, non-natural information within a
single sense has also been prominent among suggested causes.

> Although a large contingent of researchers believe the cue
conflict theory explains simulator sickness, an alternative
theory was reviewed as well. Forty factors shown or believed to
influence the occurrence or severity of simulator sickness were
identified. Future research is proposed.

Vection

> One phenomenon closely involved with simulator sickness is
that of illusory self-motion, known as vection. Kennedy et a1.
(1988) stated that visual representations of motion have been
shown to affect the vestibular system. Thus, they conclude that
the motion patterns represented in the visual displays of
simulators may exert strong influences on the vestibular system.
Kennedy, Berbaum, et al. (1993) stated that the impression
of vection produced in a simulator determines both the realism of
the simulator experience and how much the simulator promotes
sickness. They suggested that the most basic level of realism is
determined by the strength of vection induced by a stimulus. For
a stimulus which produces a strong sense of vection,
correspondence between the simulated and real-world stimuli
determines whether or not the stimulus leads to sickness.
Displays which produce strong vestibular effects are likely
to produce the most simulator sickness (Kennedy, et al., 1988).
Thus, Hettinger et al. (1990) hypothesized that vection must be
experienced before sickness can occur in fixed-base simulators.

> While viewing each of three 15-minute motion displays, subjects
rated the strength of experienced feelings of vection using a
potentiometer. In addition, before the first display and after
each of the three displays, the subjects completed a
questionnaire which addressed symptoms of simulator sickness. Of
the 15 subjects, 10 were classified as sick, based on their
questionnaire score. As for vection, subjects tended to report
either a great deal of vection or none at all. In relating
vection to sickness, it was found that of the 5 subjects who
reported no vection, only 1 became sick; of the remaining 10
subjects who had experienced vection, 8 became sick. Based on
their results, Hettinger et al. concluded that visual displays
that produce vection are more likely to produce simulator
sickness. It is also likely that individuals who are prone to
experience vection may be prone to experience sickness.
It was mentioned earlier in this report that a wider field-
of-view produces more vection and, thus, is believed to increase
the incidence and severity of sickness (Kennedy et al., 1989).
Anderson and Braunstein (1985), however, induced vection using
only a small portion of the central visual field and 30% of their
subjects experienced motion sickness.