A target in space and in an atmosphere is a ocean width in difference. No friction heat to defuse radar and/or other targeting mechanism, the complete opposite for RV where radar would not be much use due to diffusion making it difficult to maintain acquisition at early stage in re-entry.
On-board processors would also have a limitation in power so it can't be too fast in processor speed. Again ABM has far less variables to make correction in terminal phase since there is no atmosphere.
I did acknowledge the caveat in my last sentence, that processing power necessary to make the microadjustments at terminal maneuvres relies on the quality of the seeker detecting the target. But my comparison with ABM was to point out that the speeds of a vehicle and its target are not an inherent bottleneck for onboard processing -- or rather, they are not the only one.