Because the lightspeed is Mass ( gravity) and time related.
Only tiny differences can be measured such as:
in Michelson and Morley
O'Çonnel effetcs,
Babcock and Bergman experiments,
Venus- Earth reflection time residuals,
Dual CHARM satellite speed outliers,
and future experiments , see below.
On the variable lightspeed
Testing proposals for Human Free Will in the Raspberry Multiverse, Local Entanglement and M&M lightspeed.
http://vixra.org/pdf/1811.0125v1.pdf
The O‘Connell effect in eclipsing binaries explained by mass related light speed extinction distances (LASOF) of stars and even planets.
http://vixra.org/pdf/1409.0164v2.pdf
Reconciliation of GR and QM by a New Gravity-Black Hole and Lightspeed Model Called Quantum- FFF Theory.
http://vixra.org/pdf/1402.0132v1.pdf
The Semi Relativistic Higgs Field Aether with mass related lightspeed adaptation.
http://vixra.org/pdf/1310.0059v1.pdf
Experiments to determine the mass related Lightspeed extinction volume around the Earth and around spinning objects in the Lab.
http://vixra.org/pdf/1102.0056v2.pdf
