I finished the series several years ago, and I am now listening through it again. I just re-completed the fourth book, and the sequence that made me ask this question of myself is when:
Nynaeve goes toe-to-toe with Moghedien in the Panarch's palace. It seems to me she took on considerable extra risk to secure the seal.
At the end of the very first book:
Moiraine finds a broken seal at the bottom of the Eye of the World.
This early discovery implies (to me at least) that the seals can be broken purely by the Dark One's influence, with no outside intervention. No one at the time (or even by the end of the fourth book) has reached any sort of conclusion that they are weakening, let alone that they can be broken by any normal means.
It is understandable that they wouldn't want to explore the subject in great detail; accidentally breaking one could have disastrous consequences. But based on the information available, there is virtually nothing they can do to protect them either. Perhaps the Forsaken know more about how they can be destroyed/protected, but certainly not anyone on the side of the Light.
Several times in the WoT series, the protagonists and antagonists alike make concerted efforts to secure the seals in their possession. Why?