This is intentional. As you have seen, while there are quintics which can be evaluated quickly, there are also those which can take ~ 7 minutes to solve. Surely, we don't want the user to wait for 7 minutes to get a solution? This way, we can document this and if the user wants, then he can go for exact solutions knowing full well that the solution may take a long time to compute.
Also, another reason is this: Suppose that the user has a script. In that script somewhere, there's a solvable quintic. So, to the user, the script may seem to be hanged because of the quintic. I'm sure that won't be a good thing as far as the user satisfaction is concerned.