Notice that when you step on these things they sink about a quarter inch. And, it takes a little time for them to make a full stroke. So, if you walk fast enough, they won't do a full stroke. Also, the rate of sink probably depends on the weight of the person stepping on them. That rate is important for any type of electromagnetic generator. So, a skinny person walking fast won't generate as much as a fat person walking slowly. A youngster might not generate anything at all.
I can also imagine that the sinking could make them uncomfortable to walk on and might be a tripping hazard.
Finally, these are not really using kinetic energy. They are using the potential energy of a human body dropping about one quarter of an inch. By my calculation, a 100 pound person dropping one quarter inch releases about .0008 Watt hours of energy. That ain't much. And no conversion is 100% efficient.
Of course, I could be completely wrong and maybe in 10 years every street and sidewalk will be paved with these things.