Think about such a system near central train stations. You arrive at the central station by train. You hop on that thing and are transported near to your workplace.

Except you would need a path going to every workplace in every direction, and an additional unit every 100m or so per path. Plus, if these are outdoors then they have to be built heavier with more weatherproofing and will require more frequent maintenance to deal with exposure to sun, wind, rain, humidity, temperature changes, dust, trash, vandalism, etc. The entire length of each path will need to be cleaned regularly, so that’s additional staff and equipment cost.

Eventually someone will attempt to move a pallet of bags of concrete mix or bricks or kegs of beer or something. Presumably you don’t want to deal with paying an attendant to monitor every entry/exit point and all of the related employment issues, so either the unit has to be built to handle such situations and maintained against the extra wear-and-tear, or some other sort of usage enforcement has to be put in place.

In contrast, the bus has an attendant who also operates the machine, and can cover a lot more distance using only one motor and drive train system. Loading a pallet of bricks onto a bus would be highly impractical, so nobody tries to do that, so that problem never comes up. Plus, the bus can travel faster than anyone could comfortably handle on a moving walkway. Also, the bus can be driven into a garage whenever maintenance is required, so tools, spare parts, and expert technicians don’t have to be brought to it, it can go where the service is.

And beyond even that, the bus can be rerouted as the city’s needs change. The moving walkway has to be uninstalled and reinstalled in order to change its service area. The walkway is very inflexible.