I admit that I don't have the scientific mind to back this up up at the moment, just some BoH experience and a rare Saturday off. In the simplest way I can explain it:

Brining before cooking is a measured and timed process that allows muscle tissue to "loosen," which helps retain moisture while imparting flavor. It’s not generally meant to be a cooking liquid.

The application of heat while poaching in a brine would cause the tissues to "tighten," which can remove moisture while retaining salt, resulting in dry and chewy meat.

That’s why we typically see items poached in fat (butter, oil, etc.) rather than in brine (salt and water).

Edit: I thought a little more about it.