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Question is basically as is in the title. After simmering for hours, I would expect beef or pork to look fully cooked and grey in the centre, but it'll come out with some pink in the middle. Since its internal temperature should have reached around boiling, why would the inside still look like a medium steak?
(This question isn't about food safety, more about why this happens.)
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.
Note that color is not a reliable indicator of level of doneness in all cases. Pink can mean a medium steak, or it can mean a cured piece of meat or other affects.
One instance when pink does not mean medium cooked or under cooked is in some slow cooking methods, especially when things like root vegetables are included. Nitrates can be released and react with the meat, locking in some oxygen and allowing it to retain some pink color even when cooked to the point of breaking down connective tissues. Even at lower levels than are used in curing items like ham, bacon, etc, nitrates can cause meat to retain various shades of pink or at least not turn fully grey. Corned Beef for instance is has higher levels than anything you would be getting as described, but think about it. It really does not matter how long you cook it as a simmer or boil, it will retain that cured pink color unless you do something like sear it like in a hash. Even then, pieces that are big enough to cut would still show pink inside. Even without any nitrates added, some may still be there and other similar reactions in can cause some pink to remain without it indicating the level of cooking.
