A big problem in science in general is reproducibility. There are thousands of papers being published every month with peer review, which prunes out the most obvious errors, but the whole system is built on trust.

Very few labs around the world are tasked with testing results instead of trying to produce new science, and in this specific case, only google has access to the device being built and the computation tested by them impossible to verify with classic computers, unlike Shor's algorithm which is trivial to test with known primes.

Because you can verify it part of the way there.

The point is to ensure that researchers are actually doing what they think they are doing and not deluding themselves. Its not meant to prevent outright fraud.