Pretty sure engineers don't sign off on things like deck extensions, but I could be wrong.
Credentials are insufficient for all of those. A credentialed plumber or electrician could flood or burn down your house and it might be hard to figure out the root-cause, so the credential slips. You still have to do due diligence to find competent people.
I'll admit that for certain things which are easy enough that you can write down procedures for them that a credential can be valuable, but there are a reasonably small number of those things in the world and even when you do write the procedures down you're usually significantly constraining the type of project that individuals in that field can undertake. That constraining is a very important trade-off to consider when thinking about whether a credential is helpful.
Credentials are insufficient for all of those. A credentialed plumber or electrician could flood or burn down your house and it might be hard to figure out the root-cause, so the credential slips. You still have to do due diligence to find competent people.
I'll admit that for certain things which are easy enough that you can write down procedures for them that a credential can be valuable, but there are a reasonably small number of those things in the world and even when you do write the procedures down you're usually significantly constraining the type of project that individuals in that field can undertake. That constraining is a very important trade-off to consider when thinking about whether a credential is helpful.