Because in FET it's not imperceptible, it's huge. Allegedly. Somehow. Magically.
Yet strangely this change is actually imperceptible. Hmm.
Here's the simple version for you:
Do the stars and other celestial bodies exert a gravitational influence on the surface of the Earth?
RET: Yes
Some flavours of FET: Yes
Is that influence measurable using modern equipment?
RET: Stars: No Moon/Sun: Yes
Some flavours of FET: Stars: Yes Moon/Sun: Yes