My old biology teacher used to say, "evolution only works as well as it needs to." Rabbits digestive systems are so inefficient they have to eat their own shit just to get enough nutrients. Hyena clitoris are so large they sometimes suffocate their offspring during birth. You're mouth is full of vestigial molars that will likely require surgery in your lifetime. None of those things matter, as long as your genes are successfully being passed down effectively

Panda's have a digestive system that's not well suited to their diet, and they've adapted to that mostly through behavioral changes. Since they don't have kind of stomachs that efficiently digest plant matter (like a cow's four-chamber stomach), they're constantly hunting for different types of bamboo to get the nutrients they need. They eat young bamboo shoots of one species in the spring, then migrate to higher elevations to get the shoots of another. Both shoots lack calcium, so they migrate again in late summer to get more mature plants calcium-rich leaves.

One weird physical adaptation they've developed is in their pregnancies. They mate in the springtime, but fetuses require lots of calcium to develop, so females embryos basically get, "paused," neither developing or dying, until later in the season when they have more calcium in their diet.

Anyway, I guess my point is that evolution did fix the pandas digestive system to work with plants. It's just that, like most of evolution's fixes, it's a solution that's barely held together by duct tape and hope, and it could fall apart at any minute.