Are lawyers not able to guarantee a victory to a prospective client?

Of course not.

Don't be silly.

It's not possible to guarantee a victory in any case. Ultimately, it's the court who decides who wins, not the lawyer, so unless your lawyer can mind-control the judge or something to that effect, he can't guarantee what will happen.

However, if you have a really strong case, then your lawyer would be able to tell you that you have a good chance of winning, so he could tell you that you were probably going to win. Of course, it does happen that strange things happen in the courtroom, and more than once it has happened that they issue verdicts that you would think no sane judge or jury could ever reach, but there you go.

Also, lawyers will often not take on a case unless they think there is at least a reasonable chance of success, especially if they have a "no win, no fee" policy (these lawyers usually only take cases they think they are going to win). They may take on cases that you are unlikely to win, although they should advise you of this fact and realistic options for dealing with the case- for instance, a lawyer might represent a client in a case they know they cannot win, but might still aim to minimise the amount they have to pay out, for instance.

Nancy Pelosi guaranteed Donald Trump wouldn't be the next president. She said we could even take it to the bank.

If you can't believe Nancy Pelosi's guarantees, whose can you believe?

You are correct, they cannot guarantee a win, even when things look great. An excellent case, however, is excellent. Much better than a 50/50 chance or a poor chance. I don't think you can do better than having an excellent case.

Can you guarantee who will win each NFL game this week? No? Any given Sunday!

And the same applies to lawyers and doctors. Why do you think they call it "practicing" law or medicine?

a guarantee would be unethical