Your assumption about a temperature "near boiling" is wrong. Meat is an effective temperature insulator.
Meat in a pot always ends up having a heat gradient, with the center being colder than the outside. A thin piece of meat will end up the same temperature as the water after many hours, but a big slab will still have a few degrees Celsius difference.
I am also pretty sure that your meat doesn't look "like a medium steak". Sure, the color may not be grey, but the color needs very high temperatures to turn grey, maybe above 80 Celsius, while a steak stops being medium if it reaches 57 Celsius. There is a vast range between "medium" and "grey" where the color may still be in the pink part of the spectrum, but the texture is entirely different (and the color is also not the exact same shade, if you look closely). Combine this with the fact that simmering water is not close to boiling, it is 90-ish Celsius, and that stewing is done with meat which is high in isolating fat and connective tissues, and it is no wonder that, even after a couple of hours, the temperature has not compleltey evened out throughout a slab of meat and the middle stays a different color.