Why did Google not use an NP problem for their quantum supremacy experiment?
there exist problems that are hard to solve, but for which it is easy to verify the validity of a given solution: the so called NP problems.


This statement is wrong. There are many NP problems which are easy to solve. "NP" simply means "easy to verify". It does not mean hard to solve.

What you are probably thinking of is NP-complete problems which is a subset of the NP problems for which we have very, very good evidence to think they are hard. However, quantum computers are not expected to be able to solve NP-complete problems significantly more "easily" than regular computers.

Factoring is also thought to be hard, but the evidence for this is only "very good" and not "very, very good" (in other words: factoring is likely not NP-complete). Factoring is one of very few natural problems which falls in between not being NP-complete and not being easy.

The list of problems that we know that are easy to verify, easy to solve on a quantum computer but hard classicly, is even shorter. In fact, I do not know of any problem other than factoring (and the very closely related discrete logarithm problem) with this property.

Moreover, any easy to verify problem would likely have the same issue as factoring: $53$ qubits is not that many, and $2^{53}$ is huge, but just within reach of classical computing. $2^{53}$ less than $10^{16}$, and most classical computers can execute on the order of $10^9$ operations per second. We could run through all possibilities in about $1/3$rd of a year on a single classical desktop computer.

Quantum computers have very few applications which they're known to be good at, and are essentially useless for most hard NP problems.