I can see you didn't click the link otherwise you will have read the section about that.
"They" didn't solve that problem, it is not the solution to all problems in finances. So, whats your point? Vaccines does not prevent 100% of infections, does that means that they are worthless?
Which section are you referring to? The one that says "Coding errors and hacks are common in DeFi"?
You said that the contract, i.e. the software, is immutable. This means that it can't be patched. If it can be patched it's not immutable, excepting semantic trickery like each version of the contract being immutable while what "the contract" points to can be changed in order to patch it.
"They" didn't solve that problem, it is not the solution to all problems in finances. So, whats your point? Vaccines does not prevent 100% of infections, does that means that they are worthless?
Nirvana fallacy: https://en.wikipedia.org/wiki/Nirvana_fallacy