There's somewhat of a parallel that can be drawn here with the top clubs' search for world class keepers and NFL teams perennially searching for franchise QB's. Not equating the importance/weight of the position to the success of the team, but, from talent acquisition standpoint.
They don't grow on trees. The ones that are out there, are under contract, and aren't going anywhere until they are well past their primes. You just have to scout young, and hope you hit a home run with a good prospect.