Why RBF is gridless and kriging is not?
Kriging IS gridless. In fact Kriging and RBF are formally equivalent. The reason kriging tends to used on grids is more historical in the oil industry than anything else. The original papers in geostats made no mention of grids. They became popular in the oil industry because a) Flow simulation is generally done on a grid  b) upscaling is non linear (so the linear kriging change of support that was developed for mining was not applicable)..and modelling on a fine grid with flow based upscaling became the norm. c) People were more interested in simulations than estimations...and upscaling becomes even more important in this case