The phrase "are so sparse that" is correct and usable in written English.
It can be used to describe a situation where something is insufficiently present or lacking in quantity or detail.
Example: "The resources available for the project are so sparse that we may not be able to complete it on time."
Alternatives: "are so limited that" or "are so few that".
The dust not only coats the moon's surface, but floats up to sixty miles above it — as part of its exosphere, where particles are bound to the moon by gravity, but are so sparse that they rarely collide.
Life-changing experiences Strangely, for a country at the hub of Europe, Slovakia's commonly known parts are so sparse that travelling virtually anywhere is an adventure: as befits a nation more dominated by mountains or forests than almost any other on the Continent.
In spite of its high temperature, the corona yields relatively little heat, because of its low density; i.e., the constituent gas molecules are so sparse that the energy content per cubic centimetre is substantially lower than that of the interior region of the Sun.
However, the OPPW or AM-OPPW 2D OOCs are so sparse that reduce the bandwidth utilization, so the more pulses per wavelength cases should be used.
Matthew Skinner of the University of Kent, UK says fossil samples from Asia are so sparse that it is hard to infer species status.
On the other hand, we do not correct the seasonal variations in velocity change obtained from the KiK-net DCWs because the data from KiK-net records are so sparse that seasonal variations cannot be confirmed.
The image is so sparse that it almost borders on abstraction.
The heat was so sparse that the rice had been cooking for hours and was still not ready.
One is that the evidence is so sparse that people are free to frame a favourite hypothesis about what it was that made humans different.
On weekdays, the crowds can be so sparse that it feels as if these little palaces have been cordoned off just for you.
At one, Brooklyn Manor, the staffing was so sparse that a resident was put in charge of the entire place on one evening, a routine 2001 state inspection report shows.
