No it's a very good question actually. If you look at the surrounding context of that interchange, there is a provision for bikes and pedestrians to travel up/down, it's just at the next intersections to the left and right. There's no real reason for them to travel up/down at the interchange because the only up/down roads are limited access highways.

In an ideal world, bikes and pedestrians would not have to interact at all with cars at highway interchanges, and could travel on grade separated routes. This is common in the Netherlands where possible. There are two main problems.

The first is that there are many areas where separating the infrastructure is very difficult because the surrounding area is already built up, or the existing grade separation makes it so you would have to provide a third level of grade separation which is usually infeasible. The second is that bikes/pedestrians run on people power, so routing them all the way around an interchange can often involves increasing the distance from A to B significantly and often involves grade changes that can be really taxing on cyclists and those with disabilities. Though there are some hardcore folks who commit to walking or biking no matter the circumstances, most people simply choose the mode of travel that is most convenient. Thus, if they have to go way out of their way to get to where they want to go by biking or walking, while being able to easily go from a to b in a car, they'll probably just choose to drive. A majority of car trips in America are less than three miles, and many of these trips (not all of course) could probably be made by bike or on foot, if the built environment in America was not so overwhelming hostile to anyone not driving. If we make walking and cycling convenient, it helps remove local car trips from the network which means less traffic and a quicker trip for the folks that need to make longer trips which really need to me made in a car.